A pronoun must agree in number with its antecedent. That is, they must both be either singular or plural. Look at this sentence:
This happens to everyone whether they choose to believe it or not.
Here the pronoun they must agree with its antecedent, everyone. But they is plural, and everyone is singular (everyone means “every single one”). Uh-oh! What to do? Unfortunately (but unavoidably), this pronoun agreement problem broaches the “singular they” debate—the name generally given to the use of they, them, their, or theirs to refer to a singular pronoun, as in “Everyone was blowing their nose.”
The singular they has been used in English since Chaucer’s time, but most grammarians have traditionally prescribed the use of the masculine pronoun: “Everyone was blowing his nose.” In recent years, however, the singular they has gained popularity because of the move toward gender-neutral language. Nevertheless, many learned language mavens still maintain that terms such as everyone and each student should be treated as grammatically singular. Even the noted English lexicographer Henry Watson Fowler, writing in the 1920s, felt that the singular they sounded “old-fashioned” and wittily demonstrated how the singular they never seems to agree perfectly:
“Everyone was blowing their nose”?
“Everyone was blowing their noses”?
“Everyone were blowing their noses”?
Proposals for gender-neutral pronouns have been made since the 1850s, but the subsequent discussions tend to go round and round and never reach a consensus. New pronouns may be invented in the future, but for now we’re all forced to deal with this English idiosyncrasy.
In the past, most of us were taught to solve this problem by replacing the “singular they” with the “generic he,” like this:
For reasons they couldn’t explain, each of them found his good time turning bad.
But these days that’s considered sexist, so we’re advised to replace he and his, etc., with he and she, his and her, etc. That solution, though, produces clumsy sentences such as:
An employee who thinks that he or she can’t be replaced rarely stops to ask himself or herself if the boss holds the same opinion of him or her.
Horrors! That won’t do.
The best solution is to write around the problem. For instance, sometimes a troublesome singular noun can be changed to the plural. Then a sentence that says
Everyone hopes that they will win the lottery.
Many people hope that they will win the lottery. And sometimes you can ditch the pronoun. Then a sentence that says
Each guest should bring their own knife and fork.
Each guest should bring a knife and fork.
And the lottery sentence becomes
Everyone hopes to win the lottery.
Including me. Then I wouldn’t have to deal with this problem.