For National Grammar Day, linguist Neal Whitman takes a look at a long-standing source of contention among grammar enthusiasts: singular they. (Grammar purists, prepare yourselves for some unconventional rules!)

When my son Adam was doing a worksheet on words prefixed with dis-, one of the questions was, "What might cause you to distrust someone?" Adam wrote, "If they let you down." He looked at what he'd written, then said in a tone of mild wonder, "Sometimes they can be singular."

I was proud of him, not only for having absorbed his teacher's lesson on pronouns, but  for realizing that it didn't completely match what he knew about his language. Unfortunately, I couldn't leave well enough alone. The next day, I asked him, "If they is singular, then can you say, 'they is ready'?"

"Oh!" Adam said. "So are can be singular, too!"

I should have kept my mouth shut.

I'm all for ditching the rule against singular they, for reasons you've undoubtedly read before (maybe even around these parts, from Margaret Hundley Parker or Anne Curzan). I won't go into all those reasons now. But I have to admit that a categorical ban is easier to present in a grammar textbook than a set of rules that tries to cover subtleties like whether are can be singular. Furthermore, such rules are irrelevant to writers whose audience will take singular they as a sign of ignorance. That's why Garner's Modern American Usage, which acknowledges the utility of singular they, says more about how to avoid it than how to use it.

Still, in a world where singular they was unquestionably standard, what kind of rules for its use would grammar references give? Taking guidance from the Cambridge Grammar of the English Language and the Merriam-Webster Dictionary of English Usage, I offer my version here, as a list of seven do's and don'ts.

DO use they to refer back to a pronoun such as anyone, someone, everyone, no one, or who when the sex is unknown. The same goes for noun phrases such as any employee, every student, which contestant, etc.

For example: Everyone did their best. This is the least controversial use of singular they. People argue in favor of it by pointing out that words like everyone are notionally plural, but that's not really what's going on. After all, someone, anyone, and who don't seem to imply more than one person, and no one certainly doesn't.

Linguists prefer to say that they is acting as a bound variable. That's because if you put a sentence like someone thinks they got cheated into quasi-logical language, it would be something like, "There exists a person x such that x thinks that x got cheated." X is the variable corresponding to someone and they in ordinary English.

DO use they when referring back to nouns of different gender joined by or.

For example: John or Marsha thinks they can do it. This is another of the less controversial cases, since they is almost the only solution here. Using he makes the sentence mean "John thinks John can do it, or Marsha thinks John can do it."Making this sentence work with he or she requires a respectively, and just can't be taken seriously: John or Marsha thinks he or she, respectively, can do it.

DO use they to refer back to an individual of unknown gender, or whose gender you do not wish to reveal.

For example: My friend said they would be in town this weekend. This is useful for journalists preserving sources' anonymity, as well as a common means of talking about a girlfriend or boyfriend if you don't want to admit to having one. This usage is less accepted, but is useful enough that I'm coming down in favor of it.

DON'T use they when the context makes it clear that you're talking about members of a specific sex. Use he or she instead.

In a Language Log post from 2004, Geoff Pullum shares an example he heard: I think if someone in my class was pregnant I would be sympathetic to them. What seems to be going on is that the bound-variable use of they is becoming more general, losing the restriction that the nonspecific person be of unknown sex. Nevertheless, I'm ruling against this usage. He or she will work, and not leave some readers puzzled over the choice of pronoun.

DON'T use they to refer to a specific, named individual. Use he or she instead.

If you say, "Barack Obama said they would meet with the Dalai Lama," the they has to refer to some group of people; it can't refer to Obama. Of course, if you're referring to a specific individual, you probably know their sex, so why do we even need this rule when we have the one above? The reason is that there's a difference between if someone was pregnant I would be sympathetic to them and Barack Obama said they would meet with the Dalai Lama. Although the pregnant sentence sounds a little off, I know that they refers to the pregnant someone. In contrast, the they in the Obama-Lama sentence simply cannot refer to the intended person. If the above rule is discarded, I don't want this one to disappear by accident.

DO use a plural verb with singular they.

This is the complication I brought up to Adam, and one that opponents of singular they find most troublesome. In fact, though, there's a huge precedent for a pronoun with a singular-like meaning to go with plural verb forms. Centuries ago, thou (along with thee and thy)served as a second-person singular pronoun. Around the 1500s, the second-person plural pronoun you began to be used as a singular. It ultimately almost completely displaced thou, but there was never a time when it was OK to say "You art" and "You goest." You still took a plural verb form: You are, you go. Now those verb forms are simply considered to be second-person singular in addition to being plural. The only difference with they is that instead of displacing he, she, and it, it's living peacefully alongside them.

DON'T use themselves to refer to a single individual. Instead, use themself (gasp!).

Even those at ease with singular they are unsure about this one. Some use themselves, making the reflexive suffix match the plural form of them. Linguist Larry Horn noted this example in a New York Times article on the MTV show Jersey Shore: "A Guidette takes really good care of themselves, has pretty hair, cakes on makeup, has tan skin, wears the hottest heels." (This is also an example of singular they being used even when the referent's sex is known, since Guidettes are women.) Others, however, use the singular form of the suffix, -self, to match the singular meaning, as in this example from 1942: "One player laughing themself to death over the other half of their act can be overdone."

Surprisingly, there are even examples of themself used as a plural. An example from 1913: "[T]hose two men would do again what they had already done, ... with as little thought for themself." But maybe it shouldn't be so surprising: The word themself is older than themselves. (The same goes for yourself and yourselves, ourself and ourselves.) According to MWDEU (citing the OED), "themself was the normal form of the third person plural reflexive pronoun until about 1540, when it was superseded by themselfs and, ultimately, themselves."

Now themself is making a comeback, as one of the reflexive counterparts to singular they. I have written my rule to impose some order: themself for singular they; themselves for plural they. It could catch on — after all, Henry Fowler made a suggestion for streamlining the usage of that and which back in 1908, and 100 years later, for many speakers his suggestion has almost the force of law!

So there they are: my rules for singular they. I've tried to allow the most common and accepted usages, disallow the obviously ungrammatical ones, and strike a balance between utility and potential for confusion in the rest. They're ready to go ... as soon as singular they loses its stigma.