The verb of a sentence can be:
one word - He works in an office
two words - He is working in the office
three words - He has been working in the office
or even four words - He must have been working in the office
In the sentences above, work (works and working) is an example of a LEXICAL VERB. Lexical verbs give basic information. Every simple sentence has one lexical verb in it.
In the sentences above, the words between He and work are examples of AUXILIARY VERBS. There are not many auxiliary verbs. They are: do, have, be, can, could, will, would, shall, should, may, might, must.
They are very important because we can use them together with lexical verbs. For example:
Where [a] shall I [l] meet you?
[a] Is John [l] coming to the party? No, he [a] is not.
He [a] does not [l] like parties, [a] does he?
[a] Have they [l] finished? Yes, they [a] have.
We use auxiliary verbs to: