(auxiliary verb, taking a past participle) Used in forming the perfect aspect.
I had already eaten.
I have already eaten today.
I will have left by the time you get here.
Used as an interrogative verb before a pronoun to form a tag question, echoing a previous use of 'have' as an auxiliary verb or, in certain cases, main verb. (For further discussion, see the appendix English tag questions.)
They haven't eaten dinner yet, have they?
Your wife hasn't been reading that nonsense, has she?
(auxiliary verb, taking a to-infinitive) See have to.