Singular is singular, and plural is plural, and never the twain shall meet, as Rudyard Kipling would no doubt agree. Verbs have to agree with their subjects, not verbs has to agree with their subjects.
But as you apply this simple singular-singular/plural-plural rule, don’t be fooled by certain structures:
Sentences beginning with here and there.
These words, called “expletives,” move the subject so that it comes after, rather than before, the verb. Compare “Here are the boxes” with “The boxes are here.” Likewise, compare “There are three trends that concern me” with “Three trends concern me.” With sentences introduced by expletives, don’t fall into the increasingly common, and ear-grating, habit of using singular verbs regardless of what follows. In other words, it’s “Here are the boxes,” not “Here’s the boxes.” Likewise, it’s “There are three things,” not “There’s three things.”
They don’t change a thing. It’s “The severity of these problems is troubling,” not “The severity of these problems are troubling.” Don’t let the intervening phrase fool your ear. When in doubt, strike it out. You wouldn’t say, “The severity are”; you’d say, “The severity is,” so it’s “The severity of these problems is.”
Asides introduced by phases such as “in addition to’’ and “as well as.’’
Again, they don’t change a thing. It’s “This subject, as well as those subjects, is singular,” not “This subject, as well as those subjects, are singular.” Again, when in doubt, strike it out. Singular subjects take singular verbs.
But not always. Watch for these exceptions.
Compound subjects. Generally, when you join two things with the conjunction and, the resulting compound takes a plural verb, as in “The gerbil and the rat are in love.” But sometimes the compound elements are thought of as a single entity, as in “Profit and loss is important to every business.”
Also, when you create a compound subject with the conjunction or, the verb agrees in number with the closer element. If the closer element is singular, the verb is singular (even if the farther element is plural), as in “Either these three walleyes or that lake trout is going to be dinner.”
If the closer element is plural, the verb is plural (even if the farther element is singular), as in “Either that lake trout or these three walleyes are going to be dinner.” Got it?
Subjects and complements that differ in number. “Complements” are words connected to subjects by linking verbs, as in “That fish is a monster,” where monster is the complement of the subject fish. When subjects and complements are mismatched in number (that is, one is singular and the other is plural), the verb agrees with the subject rather than with the complement, as in “The problem is too many fish,” not “The problem are too many fish,” and “Too many fish are the problem,” not “Too many fish is the problem.”