Question

what is subject verb agreement​

Answer 1

Subject verb agreement simply means the subject and verb must agree in number. This means both need to be singular or both need to be plural.

Answer 2

In any sentence, the subject should always agree with the verb. Singular subjects take singular verbs and plural subjects take plural verbs.
Eg: He is happy.
(Here the subject is ‘he’ and the verb ‘is’, since ‘he’ is singular the verb is also singular)
They are happy.
(Here the subject is ‘they’ and the verb is ‘are’)
Exception to this rule*
The 1st Person Pronoun ‘I’ even though being singular will use plural form of the verb.

Eg: I write a letter. (Not I writes a letter)