My father-in-law, who owns three houses, constantly _______ about real estate taxes.
Which of the following options correctly completes the sentence above?

Question

Quizplus · Accepted Answer

A subject must agree with its verb in number. The subject in this sentence (my father-in-law) is singular, so we need a singular verb (complains).
The verb "have complained" would only be used with "I" as the subject, "you" as the subject, or a plural subject. 
The verb "complain" would be used with a plural subject.
The use of "complaining" does not make sense in this sentence without the presence of a helping verb.

My Father-In-Law, Who Owns Three Houses, Constantly _______ About Real