(Errors in language-teaching books drive me absolutely insane; please inform me of any that I have made myself! Thanks -- Tom)
Italian Conjugation: the Imperfect Indicative Tense
The imperfect tense of a verb denotes an action that was occurring or used to occur. This contrasts with the preterite, which indicates simply that an action occurred at some point in time.
e.g. Quando ero giovane, andavo spesso alla spiaggia. When I was young, I went/used to go to the beach a lot.
The imperfect is probably the easiest tense in Italian to make: all but a handful of verbs are regular in the imperfect, and the set of endings for the regular verbs is the same regardless of the conjugation of the verb (-are, -ere, or -ire).
Note that we only discuss the indicative mood here. There is another tense used (the imperfect subjunctive) when the statement is contrary to fact. Most uses of the imperfect subjunctive stem from translations of things like "If I had ..., I would have ...." We do not consider those here.
The stem for the imperfect indicative of regular verbs is formed by dropping -re from the end of the infinitive. The endings added to this stem are: -vo, -vi, -va, -vamo, -vate, -vano. The accent on the Loro form (e.g. parlavano, vendevano, partivano) falls on the third syllable from the last; on all the other forms, the penultimate syllable receives the stress.
parlare (to speak): (io) parlavo, (tu) parlavi, (Lei) parlava, (noi) parlavamo, (voi) parlavate, (Loro) parlavano
vendere (to sell): vendevo, vendevi, vendeva, vendevamo, vendevate, vendevano
partire (to leave, depart): partivo, partivi, partiva, partivamo, partivate, partivano
capire (to understand): capivo, capivi, capiva, capivamo, capivate, capivano
Note that the -isc- verbs (like capire, capisci?) do not take -isc- in the imperfect.
The verb essere (to be) is highly irregular in the imperfect:
essere: ero, eri, era, eravamo, eravate, erano
The accent in erano falls on the first syllable; in the other forms, it falls on the penultimate syllable.
There are a few other verbs that require special care: verbs that are contracted from other verbs take their imperfect forms from the original infinitive. For instance, bere (to drink) is conjugated as if it were bevere, from which it is derived. It is not strictly necessary to memorize these conjugations especially, as long as one remembers to use the long form of the verb when conjugating it.
bere (to drink; contracted from bevere): bevevo, bevevi, beveva, bevevamo, bevevate, bevevano
fare (to make, to do; contracted from facere): facevo, facevi, faceva, facevamo, facevate, facevano
dire (to say, to tell; contracted from dicere): dicevo, dicevi, diceva, dicevamo, dicevate, dicevano
Compounds of fare and dire (e.g. soddisfare, to satisfy; benedire, to bless) behave similarly.
The rest of the verbs irregular in the imperfect end in -rre. Some of them are given below. (This list is not exhaustive.)
porre (to put; contracted from ponere): ponevo, ponevi, poneva, ponevamo, ponevate, ponevano
trarre (to pull, drag, etc.; contracted from traere): traevo, traevi, traeva, traevamo, traevate, traevano
ritrarre (to retract, to portray; contracted from ritraere) follows the same pattern.
tradurre (to translate; contracted from traducere): traducevo, traducevi, traduceva, traducevamo, traducevate, traducevano
condurre (to lead; contracted from conducere), ridurre (to reduce; contracted from riducere), produrre (to produce; contracted from producere) and other verbs ending in -durre follow the same pattern as tradurre.
In conclusion: the only verbs irregular in the imperfect are essere, bere, fare, dire, their compounds, and the -rre verbs. This fact makes the imperfect simpler than even the present tense! If I give you any -are, -ere, or -ire verb, you will be able to give its imperfect easily. The -rre verbs are another story, but fortunately they are extremely rare.