At the most fundamental level, most English speakers understand that they should match the verb to the noun. If you have a singular subject, then the verb should be conjugated accordingly. He goes to the store, but they go to the store. Jim reads books, but the students read books. This is mostly straightforward.
Some trouble can arise, however, when you encounter words that appear to be singular, but require the “plural” form of the verb. And vice versa.
Singular-Looking Words That Act Like Plurals
The best way to understand this somewhat strange quirk of the English language is with a couple of examples. When we look at the word police, we get the impression that it is singular. We think of “the police” as a single, cohesive unit. However, the accompanying verb should typically take on the “plural” form.
Correct: The police are on the scene.
Incorrect: The police is on the scene.
This is because there really isn’t a “singular” form of the word police, unless you look at extensions of the term. The police chief is here. The police department is headquartered on Main Street. Another example is the word “people.”
Correct: The people are furious about the election results.
Incorrect: The people is furious about the election results.
Plural-Looking Words That Act Like Singular
Many subjects that you’ll encounter in school are like this. “Mathematics” is the science of numbers and space. It would be incorrect to say mathematics are a science. The same applies to terms like ethics, physics and politics, but really only when you are referring to the actual branch of knowledge or study itself. You’ll encounter a similar situation with certain diseases, like measles and mumps. They look like plurals and may have been plural in origin, but they act like singular words. Measles is common among children, not measles are common among children.
Strangely, certain noun expressions relating to measurements and amounts can look like plurals, but can act like singular nouns too: “Two cups of sugar is all you need.” Also, even if the title of a book or movie is plural, referring to that actual book or movie is always singular: The Sopranos was a great TV show.
Looks Can Be Deceiving
To make matters even more confusing, there are some terms that can be treated as singular or plural and still be correct. One such example is the word “family.” If you said the family arrives on Saturday, that’d be correct. If you said the family arrive on Saturday, that’d also be correct. And even though the word “data” is technically plural (the singular form is “datum”), it is oftentimes treated as singular too.
And then there are words that look plural, describe items that we think of as singular, but do indeed act as plurals when it comes to their accompanying verbs. The eyeglasses are on the table. The blue jeans are on sale. The scissors are quite sharp.
Let’s not forget about words that never really take on a plural form at all. Even if I get a bunch of swag from trade shows, I’d never say that I got a lot of swags. The swag is good.
So, what does this all mean? English is just a terribly confusing language, whether it’s your mother tongue or you’re picking it up as a second (or third or fourth) language. For every rule that you are taught, there are dozens of exceptions. The only way you can really learn is to read more and to write more. Practice may not make perfect, but it’s the best approximation we’ve got.