This might not seem basic, but it is. Subject/verb agreement is basic. All it means is that singular subjects take singular verb forms, and plural subjects take plural verb forms. Like this:
The tomatoes are growing well this year. (“Tomatoes” is plural, so it takes “are” as its verb form.)
This tomato shows signs of blight. (“Tomato” is singular, so it takes “shows” as its verb form.)
The thing that throws some people, though, is when there’s an “and” or an “or” in the complete subject. What happens then?
Thomas or William needs to call the realtor.
That “or” (a conjunction) causes the verb form to be singular, because grammatically there’s only one person (either Thomas or William) who has to perform the action (call the realtor). Yes, we can say “either Thomas or William needs to call,” but that changes the structure of the sentence; now “either” is the subject, as a pronoun standing for “Thomas or William,” and that defeats the purpose of this lesson. Remember, there are often many ways to say the same thing in English, and all of them are “correct.” They don’t all illustrate the same point, though.
Thomas and William need to call the realtor.
Now the subject is plural: “Thomas and William.” The verb form changes to the plural, “need.”
Now it’s time for me to crib from the excellent Chicago Guide to Grammar, Usage, and Punctuation by Bryan A. Garner (of Garner’s English Usage). I can’t say it any clearer than he does, so I’ll paraphrase from section 5.14.
As I said earlier, if the noun subjects are connected with “and,” the verb form is plural (see Thomas and William and the realtor, above). If there’s a prepositional phrase, though, the verb is controlled by the noun or nouns that are NOT part of the phrase, like this:
William, along with his partner, Thomas, needs to call the realtor. (The phrase “along with his partner, Thomas” does not combine with “William” to control the verb.)
I’m going to stop there, because it would be awfully easy to get deep into the weeds here and say nothing of much use. If you have a question related to agreement, by all means leave it in the comments and I’ll address it.