Rule 1: Subject-Verb Agreement is like a game of "Simon Says" but with words.
Rule 2: If the subject is singular, the verb should be too, unless you're feeling fancy.
Rule 3: If the subject is plural, the verb should be plural, unless you're trying to be a rebel, like a teenager.
For more rules, go here.
Or if you're feeling extra fancy, go here.
And if you're lost, go here.