Subject and verb agreement

Subject-verb agreement means that the verb is conjugated to match the person and number of the noun.

For example (for regular verbs):

He walks to the store. --> Subject: He (third-person singular), verb: walks (third-person singular conjugation)

They walk to the store. --> Subject: They (third-person plural), verb: walk (third-person plural conjugation).

NEVER:

They walks to the store. -->Subject: They (third-person plural), verb: walks (third-person singular conjugation).

they = plural, BUT walks = singular.

For regular verbs like 'to walk' in present simple tense:

I walk (first-person singular)

You walk (second-person singular)

We walk (first-person plural)

He/she/it walks (third-person singular)

They walk (third-person plural)

For regular verbs, the first-person singular, second-person singular, first-person plural, and third-person plural all look the same in present simple tense.

For irregular verbs, they look more different:

"to be" 

I am (first-person singular) You are (second-person singular) We are (first-person plural) He/she/it is (third-person singular) They are (third-person plural)