Intermediate English Grammar for ESL Learners - Robin Torres-Gouzerh 2016
Auxiliary verbs
14 Subject-verb agreement
If a verb phrase contains an auxiliary verb, it is the auxiliary verb, and no other verbal element, that must agree with the subject of the sentence. Examples with the auxiliary be follow.
Examples with the auxiliary have follow.
Examples with the auxiliary do follow.
No matter how complicated the verb phrase is, only the auxiliary verb form agrees with the subject of the sentence. The other elements of the phrase remain the same.