Just as certain things go together, like peanut butter and jelly or fish and chips, a singular subject in a sentence has to go together with a the singular form of the verb in a sentence. And a plural subject with a plural verb, of course.  Most people have no problem in simple subject sentences, but sentences with complex subjects are where confusion often arises.  For example, we know that it’s incorrect to say “Chocolate are good”, or “Games is fun”.   Confusion can arise with some varieties of subject-verb combinations. 

Typically they are:
When the subject comes after the verb
When the subject and verb are separated
When “either or” or “neither nor” in used
When the subject seems plural but isn’t

Subject After the Verb

In most sentences, the subject of a sentence precedes the verb, but a sentence can be constructed in a way that the subject follows the verb, and it is still perfectly correct.
“Although Mary circulated a petition to protest the crowning of a prom queen, there is many people who refused to sign it.”  One may be tempted to think “there” is the subject of the sentence, and therefore will take a singular verb, when, in fact “many people” is the subject and requires a plural verb. The correct sentence is “Although Mary circulated a petition to protest the crowning of a prom queen, there are many people who refused to sign it.” 

Subject and Verb are Separated

When the subject and the verb are very far apart, the intervening parts of the sentence can make us lose track of what should agree with what.  “Potato chips with lots of vinegar and salt, while fine if consumed moderately, is very bad for your health when consumed in abundance.”  The subject “chips” is very far away from the verb “is”, masking the fact that “chips” is plural and needs the verb “are”.

Either Or and Neither Nor

In both of these constructions, you are inevitable talking about two things, so the plural verb comes to mind. “Neither Mary nor Sally are as tall as Joanne” is incorrect since, even though the sentence is talking about both Mary and Sally, both of these people are singular nouns, therefore the verb must be singular. “Neither Mary nor Sally is as tall as Joanne” is correct.

Singular Subjects that Seem Plural

There are a number of subject nouns that would appear to be plural but are actually singular. The most common confusion arises with anybody, either, nobody, anyone, each and none. “Of all of the students in the class, nobody, not even me, are excited about the new teacher.”   Since “nobody” infers the entire class, it may confuse us to think it takes a plural verb. But it is a singular noun and needs a singular verb. To be correct, we have to say “Of all of the students in the class, nobody, not even me, is excited about the new teacher.”  

To avoid mistakes of these kinds, try to pull out the subject of the sentence and match it with its verb to make sure the right subject/verb pair is being used.