What is subject-verb agreement in English grammar?

Concept. Subject-verb agreement (also called concord) is the rule that a finite verb must agree with its subject in number and person. Number is the singular/plural property of the subject and person is its first/second/third-person property. The verb form is selected to match those two properties of the subject.

Application. English marks this most visibly in the present tense: a third-person singular subject takes the -s form of the verb, while a plural subject takes the base form.

Cross-check. The competing descriptions each name a different property of a sentence, so none of them defines agreement:

• "must match in tense": tense locates the action in time and is chosen from the time being referred to. Within one and the same tense the verb still changes with the number of the subject - "she writes" beside "they write" - so tense and agreement are separate requirements.

• "the verb always comes before the subject": that describes word order. An ordinary English statement places the subject first ("The bus comes"), and verb-first order belongs to questions and inversions ("Here comes the bus").

• "the subject and verb must rhyme": rhyme is a matching of end sounds, a matter of pronunciation and verse; no rule of grammar selects word forms by their sound.

So subject-verb agreement is the requirement that the subject and its verb match in number (and person).