Grammatically Correct: The writer's essential guide to punctuation ...

GRAMMATICALLY CORRECT
the sex of the person being referred to is unknown or irrelevant.
For example, Jj a student wants to drop a class without penalty,
they must do so in the first month. The problem is, they is irrefutably
a plural pronoun, and using it in combination with a singular
antecedent (student) presents a problem in agreement. This style
is considered acceptable in speech, but not in formal writing. For
a discussion of this issue and suggested alternatives, see "Avoiding
Male-Only Pronouns" under "Writing With Sensitivity" on page 296.
• Some controversy swirls around using they when the antecedent
is one of the indefinite pronouns each, every, either, neither, everybody,
everyone, somebody, someone, nobody, no one, anybody
or anyone. Despite some of these pronouns actually being plural
in meaning, they are all technically classified as singulars. Many
people use them as plurals, however, either because they don't
realize this is incorrect or, again, because of a conscious reluctance
to use he when both males and females are involved. Consider the
following sentences:
Each poem and essay in her scrapbook had their own page.
After the storm, every tree and bush looked as if they had been coated
with glass.
Each participant was asked if they could bring a snack.
Everyone should stay for the presentation, unless they have an urgent
deadline.
Nobody may enter until their ticket has been validated.
Someone has lost their wallet.
The first two sentences are outright wrong: The first should read
its own page, and the second, it had been coated with glass. The
remaining sentences, however, are not so clear-cut. Strictly speaking
they should take he and his, but many writers prefer to avoid
this. Substituting he or she and his or her often sounds clumsy.
Increasingly, using plural pronouns with these words is gaining acceptability.
Your own decision on how to handle these constructions
should depend on the formality of your writing and the expectations
of your audience.
For further discussion of these pronouns, see "Problem Pronouns"
under "Agreement Between Subject and Verb" on page 217.
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