When writing in the present tense, we must make sure that a singular subject is partnered with a singular verb and a plural subject with a plural verb. Matching subjects and verbs sounds easy, but sometimes it can be confusing. One common source of confusion is a compound subject.
The word compound means "composed of two or more parts." A compound subject contains two or more parts joined by a conjunction such as and or or (or both…and, either…or, neither…nor). Compound subjects may take either a singular or a plural verb, depending on the conjunction joining them. Let's take a closer look.
As we know from math, 1 + 1 = 2, which is plural. So if we take two subjects and join them with and or both…and, the compound subject is usually plural and takes a plural verb:
The hiker and the cyclist enjoy the great outdoors.
Here, the compound subject contains two singular subjects (hiker + cyclist) joined by the conjunction and. Together, hiker and cyclist form a plural unit, taking a plural verb (enjoy).
Both Gina and I are planning a trip to Sicily.
In this example, the compound subject consists of two singular subjects (Gina + I), joined this time by the correlative conjunction both…and. Together, Gina and I form a plural unit, taking a plural verb (are).
However, it sometimes happens that two subjects joined by and form a singular subject. How can that be? Well, here's one example:
Drinking and driving is dangerous.
When we say "drinking and driving," we are talking not about two separate activities but about two actions combined into a single activity. Together, the words drinking and driving form a single unit and so must take the singular verb is.
Here's another example:
The senior writer and editor was promoted to the position of editor-in-chief.
Here, the words senior writer and editor refer to the same person, forming a unit that takes the singular verb was.
If the parts of a compound subject are joined by or, either…or, or neither…nor, the verb agrees with the part nearest the verb:
Atsuko or Sam is bringing sushi.
Either Atsuko or her parents have made rice.
Has either Atsuko or her parents made rice?
Neither Sam's parents nor his sister has brought noodles.
Neither Sam's sister nor his parents have brought noodles.
For more on the basics of subject-verb agreement, read Basic subject-verb agreement.