What Are Compound Subjects in English Grammar?

A compound subject is a subject made up of two or more simple subjects that are joined by a coordinating conjunction (such as and or or) and that have the same predicate.

The parts of a compound subject may also be joined by correlative conjunctions, such as both . . . and and not only . . . but also.

Although both parts of a compound subject share the same verb, that verb is not always plural.

Examples and Observations

• Dave and Angie own a new Honda Accord, but they prefer to drive their old van
• Wilbur and Orville Wright ran a printing business from their childhood home, and as young men, they operated a bicycle shop.
• "My uncle and my cousin are both lawyers, as my father was."

Agreement With Compound Subjects

"Normally a subject made up of more than one element takes a plural verb ("The President and Congress are at loggerheads"), although occasionally, when the elements add up to the same idea, the verb is singular ("The wear and tear on the car was tremendous"). But focus an eye on these compound subjects followed by singular verbs, all of which are correct:

• Everything in the cupboard and everything on the table was smashed.
• Everybody favoring the plan and everybody leaning toward it were interviewed.
• Nobody in my house and nobody on my street has been robbed.
• Anyone who has read the book and anybody who has even heard of its ideas agrees with the author.

Compound Subjects Joined by Or or Nor

"Unlike subjects joined by 'and,' the very role of 'or' and 'nor' is to separate, to tell us that it's not both things, but one thing or the other that the verb applies to. So the rule is:

• Subjects joined by or or nor are not considered as a group, and the verb's person and number should agree with those of the subject's individual parts.
• There are three possible scenarios here. If both parts are singular, as in the subject Mary or Donna, then the verb is singular. If they're both plural, as in the subject Neither the girls nor the boys, the verb is plural. In really tricky sentences where you have one of each, such as Either Tony or his daughters, the verb should agree with whatever part of the subject it's closest to in the sentence; for example, either Tony or his daughters are or either the daughters or their father is. 

Sources

David R. Slavitt, "Conflations." Short Stories Are Not Real Life. LSU Press, 1991

Ann Batko, When Bad Grammar Happens to Good People. Career Press, 2004