An unaccusative verb is one that underlyingly has a
direct object but no
subject. Direct objects normally take
accusative case, but there appears to be a universal restriction that verbs without subject roles can't assign accusative: this is known as
Burzio's Generalization. So the direct object has to move up and become subject.
All sentences have a subject, at least in English, and in many other languages. It might be a requirement that all clauses in all languages have subjects at some level: this is known as the Extended Projection Principle (EPP). Many languages can lack subjects on the surface, such as Italian, Spanish, Japanese, and Chinese; these are posited to have a phonetically silent element pro in subject position. Other languages may use expletives to fill in the subject position when there is nothing overt there: English uses the words it and there for this purpose.
(1) John burnt the house down.
(2) The house burnt down.
(3) It seems the house burnt down.
(4) There is a burnt-out house next door.
(5) There arrived a man.
(6) A man arrived.
(1) is
transitive, (2) is
middle voice, (3) to (5) use
expletives. The unaccusative ones are (5) and (6). The underlying structure is
arrived a man, and since this lacks a subject, it has to be realized by putting something in
subject position.
Subject and object are structural notions, and will change depending on how the sentence is organized. Contrastingly there are semantic roles , called θ-roles, so that 'the house' is PATIENT of the verb 'burn' in all of (1)-(3), the one suffering the burning, however it is expressed.
A person who arrives is not actively doing anything; they are undergoing a motion, and this θ-role is known as THEME (origin of the θ in the name). A THEME is inherently object-like, as in 'Mary gave the book to John', where the book is THEME. In many languages, such as Italian, French, or Dutch, verbs of motion take a different auxiliary for the perfect tense: most verbs take 'have' as in English but motion verbs take 'be' (Jean est parti; Maria è arrivata). The superficial subjects of these unaccusative verbs are underlyingly themes that have had to move to fulfil the EPP.
The passive voice is analysed in a similar way. The book was stolen is regarded as coming from was stolen the book, where the THEME is 'the book' and the passive morpheme makes the verb unaccusative. Since the unaccusative can't assign accusative, the THEME has to move up into subject position to receive Case marking, and in that position it gets nominative Case. In languages like Italian and French, passives also take the auxiliary 'be', like verbs of motion.
Unaccusatives are intransitive verbs: they end up with only a THEME or PATIENT subject. An intransitive verb that has an AGENT for its subject, someone actually doing something, is called unergative: an example is Mary ran. Although this is a verb of motion, it does not take 'be' in French. Etre is used only with change-of-state motion verbs like entrer, montrer, descendre, aller, partir, but not with active forms of motion such as courir, marcher, danser, sauter.
In English we see another difference between unaccusative and unergative verbs with adjectives indicating a result. You can attach such an adjective to verbs with a THEME or PATIENT, whether it's the object (They hammered the metal flat) or the subject (The river froze solid), but not to an AGENT: you can't say *Mary ran tired meaning 'tired' as the result of running. (There are some idioms that express this resultative sense as a small clause, attaching the adjective to a noun or pronoun: run herself ragged, shout oneself hoarse, drink the cup dry, but in these cases not the subject but the object is the PATIENT.)