The rules are:
1. The verb must agree with its subject. Whether its singular or plural. Plural subject must have a plural verb.
Ex. The BOY WALKS at the park

I forget it... but I hope it will help you.... sorry

SVA or Subject Verb Agreement. For a correct grammar the subject must always go with the verb.
If the subject is singular the verb must be plural or with 's' ex.
The boy walks at the park.
The girl whips her dog.
If the subject is plural the verb should be singular or without 's' ex.
The trees sway with the wind.
The dogs bite the girl.