A singular subject requires the singular form of a verb. A plural subject requires the plural form of the verb. Makes sense, right?
Just in case you get stuck, we at Shmoop have a tip to keep in the back of your brain—you know, like where you keep all of the state capitals and the lyrics to "I Wish" by Skee-Lo.
It's called the "One–S" rule, and here's what it means:
In most sentences, either the verb has the –s ending or the subject has the –s ending. "One–S." Not two, not three, not forty-two. Only one.
If, when you look at your freshly typed sentence, both the subject and the verb end in –s, then there's a decent chance that your subject and your verb disagree, and you'll need to send them to mediation or couples counseling.
Or, you know, just take the –s off of one of them.