In a sentence in the Simple Present tense, a single noun subject takes a singular verb (almost always ending in “s”).
However, when they are plural, subjects take plural verbs (almost always the “normal” present tense verb that you would use after “to,” often not ending in “s”).
One way to remember is that if you are using the Simple Present tense, your S needs to go somewhere: either on the noun or on the verb. Another way is to remember that if your subject is Singular, then your verb needs an S.
If your subject is a noun, this only occurs when using Simple Present tense or when using “was/were.”
What happens when you join singular subjects? Does that count as singular or plural? Well, with multiple subjects, verb agreement depends on the whole compound subject together.
Conjunctions that “add” the subject elements together, such as “and,” almost always mean the compound subject needs a plural verb. (1 cat and 1 cat together make up a group of 2 cats.)
On the other hand, conjunctions that treat the subject elements as “separate” such as “or” and “nor” almost always mean the compound subject needs a singular verb. (1 cat on its own kept separate from another cat on its own does not make up a group of 2 cats.)
However, if one or more of the subjects is plural, the conjunction doesn’t matter. Go ahead and use a plural verb. (2 cats are always plural whether they are on their own or mixed with another group of 2 cats to make 4 cats.)