Probably the most commonly described division of verbs is into the classes of transitive and intransitive verbs. Transitive verbs take an object ('buy' is transitive: we say "I bought some bananas", but not *"I bought".); intransitive verbs do not take an object ("They died", but not *"They died him".)

In fact, most verbs in English can be both. We can say "I woke up" and "I woke her up", "They ran" and "They ran the marathon". Verbs that can be both transitive and intransitive are called ambitransitive. Given that so many English verbs are ambitransitive (you might have noticed that 'disappear' is now also used transitively: "The Sandinistas disappeared him."), it might make more sense to say that transitivity is not a feature of verbs themselves. Rather, a verb is used transitively or intransitively.

Now, ambitransitive verbs are quite simple. We form the transitive verb from the intransitive verb just by adding an object. No real mystery there. Ergative verbs have an extra property, however. Take the sentence: (1) The meeting opened at 12:00.

Logically, the meeting actually did nothing: to be precise we could say (2) Someone opened the meeting. If we want to focus on the meeting we could use the passive: (3)The meeting was opened at 12:00.

But open is ergative, allowing us to recast the object of the verb used transitively (the meeting in sentence (2)) as the subject of the verb used intransitively (Which is (1)).

This is a structural rule: the object of an ergative verb used transitively is the subject of the ergative verb used intransitively. This is quite a different situation to other ambitransitive verbs.

Sentence (1) is thus active in structure but passive in meaning. The effect is to attribute the action to the object rather than the real agent. In most sentences, the grammatical subject is the agent of the verb. Ergatives allow a certain metaphorical effect, focussing on the process that the object indergoes as if the object were actually performing that process. Consider: The ice caps are melting (is this something they are doing themselves, or are they being caused to melt?), Prices are increasing (but actually people are increasing the prices.)

Very many verbs of process are ergative. If the cause of the action is irrelevant or unknown, or if the writer wishes to avoid apportioning blame to the agent (Petrol prices are increasing sounds very different to Petrol companies are increasing their prices), ergatives are useful.