Auxiliary Verbs

In CGEL, an auxiliary verb is a verb that does not head the lexical predicate, but instead marks grammatical properties of the clause and combines with a non-finite VP.

Core points (only what’s relevant here):

Auxiliaries express tense, aspect, modality, or voice

They precede a lexical verb in a non-finite form

They cannot occur as the sole lexical predicate in the clause (except be as a copular verb, which is a different use)

We were reading the book

We → pronoun, subject

were reading the book → predicate, realised by a VP

were → auxiliary verb (be), marks progressive aspect, selects the non-finite VP reading the book

reading → lexical verb, head of the VP, predicates the event of the subject

the book → object NP, selected by the lexical verb

Predicate: were reading the book (VP, headed by lexical verb reading)