what are the rules in subject-verb agreement?

Answer 1

Singular subject gets a singular verb and plural subject gets a plural verb.
Singular subj.-verb examples:
The dog runs after the cat.
The cat chases its tail.

Plural subj.-verb examples:
The DOGS RUN after the cat.
The CATS CHASE their tails.

IN the case of pronouns as subjects, I and you will take the plural verb, along with they.
He, she, and it take the singular verb.

Hope this helps some?
Here is one of many websites that can help you more with this topic:
http://grammar.ccc.commnet.edu/grammar/sv_agr.htm