Likewise, pronouns agree with their antecedents in number:
In the first case, the underlined pronoun at the end of the sentence, and its underlined antecedent at the beginning, both are plural; in the second case, they are both singular. (The antecedent is the noun for which a pronoun serves as a kind of stand-in.)
One of the most common violations of the rule of agreement in number comes about when people use the plural personal pronoun they (or them or their), and sometimes plural verb forms to match, when the antecedent and/or subject is grammatically singular, but refers to a person in the abstract rather than to a specific person. Such a subject is called “indefinite.”
People do this because they want to avoid specifying gender by choosing between the masculine singular pronoun he/him/his on the one hand, and the feminine singular pronoun she/her on the other. For the modern feminist movement has made it socially unacceptable to follow the formerly standard practice of using the masculine pronoun as generic (as in “Every student should bring his book to class”); and the gender-neutral pronoun it/its is felt to be suitable only for referring to a thing and not to a person.
Perhaps the plural pronoun they/them/their will eventually take over the function of the singular completely, just as the once exclusively plural second-person pronoun you/your has taken over the function of the now-obsolete second-person singular pronoun thou/thee/thy. At that point we shall presumably be saying and writing, “Amy brought their book to class,” when we mean that Amy brought Amy’s own book. (As of yet, “Amy brought their book to class” makes us think that some group of people owned the book, and we will likely search the context for clues to what group that might be.) And we shall presumably also be saying and writing “Iím always sorry to hear that a student has injured themself [or theirself].” Both of these forms of the reflexive pronoun themselves seem utterly barbarous now (unlike the second-person-singular reflexive form yourself); and yet themselves is too clearly plural to feel comfortable with the clearly singular antecedent “a student.” Clearly, then, they/them/their has not yet superseded he/him/his and she/her to the same extent that you/your has superseded thou/thee/thy. For this reason, using they/them/their as singular remains a somewhat marginal and controversial usage, avoided by the most careful speakers and writers.
As Alexander Pope wrote, in his “Essay on Criticism” (1709),
In words as fashions the same rule will hold,We are clearly seeing a trend towards more and more usage of they/them/their with singular antecedent; and while some will regard joining the trend as evidence of an unthinking herd mentality, others will regard resistance to it as an affectation, the linguistic equivalent of a polka-dot bow tie. Clearly, I tend to the former view; for the other, see the following:
Alike fantastical too new or old:
Be not the first by whom the new are tried,
Nor yet the last to lay the old aside.
Gracefully avoiding this problematic usage can admittedly be difficult sometimes. The “he or she” solution (for example, “every student should bring his or her book to class”) is cumbersome, and variants “s/he,” “(s)he,” and “he/she” are defective in conveying no clear pronunciation to the mind’s ear. Using the pronoun “one” tends to sound so stuffy that few students can bring themselves to use it consistently enough to be correct. (“One should always bring one’s book to class.”) Often the best solution is to recast the whole thing in the plural: “All students should bring their books to class.”