The Icelandic adjective, in concordance with the language's heavily inflective nature, declines in a wide variety of ways. Greatly unlike English, the adjective must agree in case, number, and gender, which are determined by the noun's own declension (see Icelandic noun cases and declensions). Though the methods by which an adjective's ending changes are not so complicated as that of an Icelandic noun's, they are still difficult. Aspects of Icelandic's Germanic background make its adjective declension more complex than even Latin's, a language usually considered especially fond of changing the ends of every word it can get its hands on. The added complexity results from a notion of strong and weak adjectives. These two terms are bandied about in a wide variety of contexts, including in English to describe verbs that do not form their past tense or past participle regularly, but in the case of adjectives they mean something different than one might expect. In any case, the practical result is that there are twice as many possibilities of declension due to the possibility of an adjective being either strong or weak depending on context. Add to that the necessity of declining according to case, number, and gender, plus a couple of different ways to do so according to the regularity or irregularity of the adjective in question, plus a whole new set of endings for comparative and superlative forms, and the result is a dizzying array of possible endings. Their application, however, is more regular than might at first appear, a small bit of solace for the learner of this ancient and complex language.

Strong and Weak

All inflectional languages of the Indo-European language (even English, on very select occasions) require agreement between adjectives and the nouns they modify. In the Romance languages, this only involves distinctions of gender and number, in Russian case must also be considered, and in German one must add to that definition or indefinition. As a Germanic language and even further a close relative to Old Norse, Icelandic is extremely precise about its adjectives. Not only must they agree with the nouns they modify in terms of case, number, and gender (quite alot, considering there are four cases, two numbers, and three genders; twenty-four possibilities in total), they also change depending on whether the noun in question is modified by a definite article or not. In other words, the adjective describing "the good man" (góði maðurinn) is different from the adjective describing just plain "good man" (góður maður). It would be good to note that the definite article in Icelandic is more commonly suffixed to the noun (the -inn ending, in this case) than preceding it. In literary language it would be acceptable to write "hinn góði maður" instead, but this is not done at all in the spoken language. The adjective used when a definite article is present is called a weak adjective. Its counterpart in the absence of a definite article is called a strong adjective.

So why make this distinction. At first blush it seems at least redundant, perhaps straight out sadistic. After all, for every case, gender, and number there also has to be a weak and strong version, bringing the possible number of endings up to forty-eight (and that's without irregularity). Despite what many may observe, languages rarely tolerate inefficiency for long. So why the extra layer of declensional complexity? Precisely because of efficiency! Icelandic, unlike English, does not depend on word order for grammatic information. One can jumble all the words in a sentence out of order, and although it would look odd, the sentence would be perfectly comprehensible because all grammatic information concerning subject, direct object, indirect object, person, etc. are contained within the words themselves. In fact, word order is often altered beyond the English standard Subject-Verb-Object to place emphasis. In order to facilitate such flexibility, all declensional endings have to provide their grammatic information precisely.

But if one examines a sample noun's declension, one notices that there are actually a number of redundant endings. A noun with the ending -ar could be, for example, weak feminine Nominative plural, weak feminine Accusative plural, strong neuter Nominative singular, strong feminine Genitive singular, strong masculine Nominative singular... you get the point. Endings do not inherently provide exact information about case, number, and gender, rather they vaguely point to it.

Something more must be done. With such redundant endings, signaling a wide variety of grammatic possibilities, how is one to determine the function of a noun in the sentence? In context, of course, it would absurdly simple to do, simply see where the noun is in the sentence and how it relates to the other words. One wouldn't even need to look at the ending. But say you're talking with someone in a snowstorm, and the noise keeps muffling and interrupting parts of your speech. The listener needs to fill in for those dropped packets, and if given the individual words sans grammatic context they'll have trouble determining whether "the dog bit the mailman with his fangs" or "the mailman bit the fangs with his dog" or "the dog with his mailmans bit the fang"... etc. It's all about efficient transmission of information. So the solution in Icelandic is definite articles. They also decline according to case, gender, and number, and they have individual forms for every single possibility. With a definite article attached, the word's grammatic context becomes unambiguous. Problem solved.

Except. One sometimes doesn't use a definite article to modify a word. Icelandic has no indefinite article like English's a/an, and in any case sometimes no article at all is appropriate. What does one do to resolve ambiguity then? An adjective, in this case a strong adjective. Strong adjectives have fairly individual endings, which when coupled with the noun's own makes its grammatic information sufficiently clear. But if a definite article's already present, there's no need to distinguish further, and a weak adjective with its fairly uninformative endings is used. Thus Icelandic (and Germanic languages in general) took advantage of its typical specificity of definition or indefinition to solve a problem of efficient information transmission. Nice.

So what about comparatives and superlatives? They're relatively simple. In both cases the root adjective has a small inflection attached which indicates it as either comparative or superlative, and that inflection is then declined according to regular adjective declension methods. Comparatives can only be weak, however superlatives can be either weak or strong. Icelandic has no equivalent to the English duplicity of using either more/most or er/est (which I bet is a bit frustrating for foreign learners) depending on the adjective, but it does have a couple different ways of attaching the ending mostly dependent on the adjective's own phonetic qualities. In other words, usually predictable. What a surprise...

A final important note. Only descriptive adjectives, those highlighting a quality like green or heavy, distinguish between strong and weak. Limiting adjectives (mostly those distinguishing a number, like 'all', 'no', or 'five') can only be strong.

Strong Declension

The strong declension of adjectives is not particularly regular. Much like the strong declension of nouns, there are several different categories of declension depending on the adjective's phonetic qualities. They are usually very similar, even recognizable (for example, the transformation of an a root vowel to ö between two otherwise identical categories is something one sees constantly in Icelandic, just take a look at how many times it happens here). Sometimes the differences are more varied, but never does the declension categories actually stray that far from each other. One last thing to note, purely for the linguistic joy of it, is that in all neuter declensions no matter strong or weak the singular Nominative and singular Accusative are the same. This is the same in German. And Latin. And Russian. And Lithuanian. And any declensional Indo-European language with a neuter gender. While the endings may be different, they all share a redundancy between singular Nominative and singular Accusative forms in the neuter gender. This is a very ancient relic from the source language of the entire family, Proto Indo-European.

Class 1
Adjectives with a masculine Nominative singular form ending in -ur (the most common masculine Nominative singular ending in nouns as well). These are the vast majority of adjectives. One will notice there are two words declined below, one of which represents the above-mentioned umlaut when an a root vowel is present. This is extremely common in Icelandic, ocurring in every imaginable form of declension. If there's a u in the ending, an a is going to change. It's quite regular. Note that the second half of the rows is reserved for plurals.

      Masc.    Fem.      Neut.
Nom.| feitur | feit    | feitt
Acc.| feitan | feita   | feitt
Dat.| feitum | feitri  | feitu
Gen.| feits  | feitrar | feits
Nom.| feitir | feitar  | feit
Acc.| feita  | feitar  | feit
Dat.| feitum | feitum  | feitum
Gen.| feitra | feitra  | feitra

      Masc.     Fem.       Neut.
Nom.| bjartur | bjart    | bjartt
Acc.| bjartan | bjarta   | bjartt
Dat.| björtum | bjartri  | bjartu
Gen.| bjarts  | bjartrar | bjarts
Nom.| bjartir | bjartar  | bjart
Acc.| bjarta  | bjartar  | bjart
Dat.| björtum | björtum  | björtum
Gen.| bjartra | bjartra  | bjortra
Class 2
This is a small collection of adjectives which look exactly like Class 1 adjectives, but are different because their root stem originally included the -ur ending, instead of having it attached. There is no way to tell that an adjective's root stem is actually the full Nominative masculine singular form except by knowing its historic etymology. In other words, they must be memorized. Similarly to Class 1, umlaut occurs when the root vowel is an a.
      Masc.      Fem.        Neut.
Nom.| snotur   | snotur    | snoturt
Acc.| snotran  | snotra    | snoturt
Dat.| snotrum  | snoturri  | snotru
Gen.| snoturs  | snoturrar | snoturs
Nom.| snotrir  | snotrar   | snotur
Acc.| snotra   | snotrar   | snotur
Dat.| snotrum  | snotrum   | snotrum
Gen.| snoturra | snoturra  | snoturra

      Masc.     Fem.       Neut.
Nom.| magur   | mögur    | magurt
Acc.| magran  | magra    | magurt
Dat.| mögrum  | magurri  | mögru
Gen.| magurs  | magurrar | magurs
Nom.| magrir  | magrar   | mögur
Acc.| magra   | magrar   | mögur
Dat.| mögrum  | mögrum   | mögrum
Gen.| magurra | magurra  | magurra
Class 3
This class is reserved for adjectives with accented stem vowels of -á-, -ó-, or -ú-. They will always end in an -r.
      Masc.    Fem.      Neut.
Nom.| trúr   | trú     | trúrt
Acc.| trúan  | trúa    | trúrt
Dat.| trúum  | trúrri  | trúu
Gen.| trús   | trúrrar | trús
Nom.| trúir  | trúar   | trú
Acc.| trúa   | trúar   | trú
Dat.| trúum  | trúum   | trúum
Gen.| trúrra | trúrra  | trúrra
Class 4
This class has a similar stem vowel qualification, adjectives with stem vowels of -ý- and -æ-. Not all adjectives with either of those stem vowels belongs to this class, if they end in -r, -s or -consonant+n they might belong to Class 5. The memorization demon strikes again.
      Masc.     Fem.      Neut.
Nom.| hlýr    | hlý     | hlýtt
Acc.| hlýjan  | hlýja   | hlýtt
Dat.| hlýjum  | hlýrri  | hlýju
Gen.| hlýs    | hlýrrar | hlýs
Nom.| hlýir   | hlýjar  | hlý
Acc.| hlýja   | hlýjar  | hlý
Dat.| hlýjum  | hlýjum  | hlýjum
Gen.| hlýrra  | hlýrra  | hlýrra
Class 5
As mentioned above, this class is for adjectives that don't fit in either Class 3 or 4 and end in an -r, -s, or -consonant+n. It is subject to umlauting just as any other class is.
      Masc.      Fen.       Neut.
Nom.| gjarn    | gjarn    | gjarnt
Acc.| gjarnan  | gjarna   | gjarnt
Dat.| gjörnum  | gjarnri  | gjörnu
Gen.| gjarns   | gjarnrar | gjarns
Nom.| gjarnir  | gjarnar  | gjarn
Acc.| gjarnra  | gjarnar  | gjarn
Dat.| gjörnrum | gjörnum  | gjörnum
Gen.| gjarnra  | gjarnra  | gjarnra
Class 6
This class consists of monosyllabic adjectives ending in -nn and -ll, as well as disyllabic adjectives ending in -ull.
      Masc.    Fem.      Neut.
Nom.| sæll  | sæl    | sælt
Acc.| sælan | sæla   | sælt
Dat.| sælum | sælli  | sælu
Gen.| sæls  | sællar | sæls
Nom.| sælir | sælar  | sæl
Acc.| sæla  | sælar  | sæl
Dat.| sælum | sælum  | sælum
Gen.| sælla | sælla  | sælla
Class 7
This class is similar to Class 6 except that the adjectives are always disyllabic and end only in -ll. The main distinguishing factor is that they drop the unstressed root word vowel in certain cases.
     Masc.      Fem.       Neut.
Nom.| vesall  | vesal    | vesalt
Acc.| veslan  | vesla    | vesalt
Dat.| veslum  | vesalli  | veslu
Gen.| vesals  | vesallar | vesals
Nom.| veslir  | veslar   | vesul
Acc.| vesla   | veslar   | vesul
Dat.| veslum  | veslum   | veslum
Gen.| vesalla | vesalla  | vesalla
Class 8
This class consists of only two adjectives: mikill (large) and lítill (small). Nice that they're at least semantically similar.
      Masc.    Fem.       Neut.
Nom.| mikill | mikil    | mikið
Acc.| mikinn | mikla    | mikið
Dat.| miklum | mikilli  | miklu
Gen.| mikils | mikillar | mikils
Nom.| miklir | miklar   | mikil
Acc.| mikla  | miklar   | mikil
Dat.| miklum | miklum   | miklum
Gen.| mikla  | mikilla  | mikilla
Class 9
This class consists of disyllabic adjectives ending in -inn. A few of them are past participles of verbs.
      Masc.    Fem.       Neut.
Nom.| farinn | farin    | farið
Acc.| farinn | farna    | farið
Dat.| förnum | farinni  | förnu
Gen.| farins | farinnar | farins
Nom.| farnir | farnar   | farin
Acc.| farna  | farnar   | farin
Dat.| förnum | förnum   | förnum
Gen.| farnna | farnna   | farnna
Class 10
This class is made up of past participles of weak verbs in the first class. These verbs can be identified by their ending, -ja.
      Masc.     Fem.       Neut.
Nom.| galinn  | galin    | galið
Acc.| galinn  | galda    | galið
Dat.| göldum  | galinni  | göldu
Gen.| galins  | galinnar | galins
Nom.| galdir  | galdar   | galin
Acc.| galda   | galdar   | galin
Dat.| göldum  | göldum   | göldum
Gen.| galinna | galinna  | galinna

Weak Declension

These are much easier to handle. There are still classes, but there are less of them. Even better, there are only four endings to each class: Nominative singular, plural, and every other case in singular and plural.

Class 1
This is every standard adjective. Those which drop a vowel during declension in their strong forms drop it universally in the weak form.

      Masc.   Fem.    Neut.
Nom.| vesli | vesla | vesla
Oth.| vesla | veslu | vesla
Nom.| veslu | veslu | veslu
Oth.| veslu | veslu | veslu

      Masc.      Fem.       Neut.
Nom.| bjarturi | bjartura | bjartura
Oth.| bjartura | björturu | bjartura
Nom.| björturu | björturu | björturu
Oth.| björturu | björturu | björturu
Class 2
These are comparitive adjectives only.
      Masc.     Fem.      Neut.
Nom.| veslari | veslari | veslara
Oth.| veslari | veslari | veslara 
Nom.| veslari | veslari | veslara
Oth.| veslari | veslari | veslara
Class 3
Adjectives which end in -a and present participles made into adjectives do not decline at all.

Comparitives and Superlatives

Comparatives and superlatives are formed through suffixes, much like the English endings er/est. The clearly identifiable form among all comparative endings is -ri, often -ari, which will always be declined weakly. The identifiable form among all superlative endings is -stur, often -astur which is declined according to Class 1 in strong and as normal in weak. Beyond this, comparatives and superlatives must be memorized. They cause umlauts in a wide variety of stem vowels, as well as consonantal changes within the adjective itself. Some comparatives and superlatives are almost completely separate words from their root positive forms. Once the form has been found, however, declension is at least easier.

Sources used:

Einarsson, Stefá, Icelandic. Baltimore: The Johns Hopkins University Press, 1945.
Neijmann, Daisy L., Colloquial Icelandic. London: Routledge, 2001.

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