Update Your Usage:
Why "his or her" should retire.
In our everyday conversations, we tend not to notice the looseness of our language; no one will likely fault you for an incomplete sentence, nor chastise that mother at dance practice who tells her child "you did good out there," when she really means the kid did well. In a casual setting, you might hear someone say something like “whoever blocked me in should move their car,” and—had you even noticed it in the moment—you would likely forgive the grammatical error. Written language, however, obeys more rigid rules. Proofreaders, English teachers and standardized tests would all like to point out the fault in your pronoun usage (which they must remind you lies not in your stars, but in yourself). In the conventions of standard English, a pronoun ( a word taking the place of a noun) must agree with its antecedent (the word it replaces) in number. This rule is not difficult to follow, since in English grammar we have only two numbers: singular, and plural. “Whoever blocked me in should move their car” suffers from faulty agreement. While we are not usually scrupulous with pronouns in our speech, in writing such recklessness invites the red pen.
When used as the subject of a sentence “whoever” is always singular. There is no room, however, for an ambiguously singular or plural "they" in grammatically correct writing; according to standard usage, “they” is an exclusively plural pronoun. So, to avoid confusion in a case like this one should phrase the sentence with “his car,” or even adopt the safer choice of “his or her car,” provided that the gender of the car’s owner was unknown.
While the conventionally correct phrasing should be “whoever blocked me in should move his or her car,” let’s take a moment to advocate for the original sentence. In this sentence, and in many like it, “his or her” just sounds stuffy. Maybe it is time to consider that using a singular or plural "they" as the situation requires should enter standard usage. We use “they” to refer to a singular subject in spoken English so frequently that we may want to stop treating this idiomatic expression as a mistake, and start calling it proper grammar. While it may vex a few hard-line grammarians, this position has some sound reasoning on its side.
English as a language does not habitually pay much attention to gender. Our list of words that correspond to a specific gender is small, and pronouns probably take the most space on that list. English oddly lacks a singular, gender-neutral pronoun, according to current standards of grammar. While we do use "it" to refer to genderless things, to use it in reference to people would be offensive. We English speakers have been using “they” as singular pronoun for centuries. It isn’t even the first pronoun to receive the plural-to-singular treatment, since “you” was for a long time exclusively plural. The singular second person was “thou”, a now outdated word. For a long while the default gender in English was masculine. In ambiguous cases, the rules preferred the use of "he", or "him" as singular pronouns. That exclusive use of the masculine is sexist. On the other hand "his or her" is an awkward solution. Using "they" as a singular pronoun should be acceptable, even if your style guide says otherwise.