When I was a middle schooler I learned that you should never use a plural pronoun with a singular antecedent. Doing so could brand you a grammar ignoramus, which for Mr. Swenson, my 7th grade English teacher, was a fate almost worse than death.
[t]he rule says that because they is a plural pronoun, it must have a plural antecedent. This means that the sentence If anyone has a problem with that, they should tell me is wrong because anyone is singular and they is plural. They should be switched to a singular pronoun, but which one? “Generic he” was the prescription in the 19th century (If anyone has a problem with that, he should tell me), but as it became clear that he was neither generic nor neutral, the suggestion was to either use the cumbersome “he or she” (If anyone has a problem with that, he or she should tell me) or to rewrite the sentence entirely (Got a problem with that? Let me know).
To avoid the “cumbersome ‘he or she,'” many of us alternated their use, as in this example: “If anyone has a problem with that, he should tell me. Even better, she should complain to the director.” This approach, though, is nothing if not awkward—and confusing. Now, however, it appears we can use “they” with impunity; the Washington Post says so, and, at the American Dialect Society’s 2015 annual meeting, “they” was named “word of the year.”
Still, I don’t know if I will be able to make the switch. In a conversation yesterday, I heard myself say something like, “Anyone can take the class if they think it will help them.” Immediately Mr. Swenson’s face appeared before me, and he was not happy.