“They” can be used as both singular term, or plural, with the context of the sentence making it clear which is being used. The same is also true for “you” which can also be singular, “Joey, you forgot your bag!” or plural “Students, you must get your books by the end of the day.”
The issue here is that many people weren’t taught the singular form of “they” in school, though it has been used in English for centuries, so there are people who feel that it sounds awkward and strange due to a lack of familiarity.
“They” has been used for centuries in both single-person and plural contexts – the reason is just that English lacks any other singular third-person pronoun, aside from “it,” which we don’t use for people. They, used for a single person, was once criticized for this single-person use (I’m talking hundreds of years ago), but has since become an accepted singular pronoun when gender is unknown, unstated, or when the speaker prefers it.
Like in the sentence “When speaking to an elder, treat *them* with respect.” It’s clearly a singular person (an elder), but the usage isn’t limited to a male or female elder.
English is weird like that – we also don’t have a plural second-person pronoun. *You* can mean one person, “Would you like to come have dinner?” or to multiple people “Good evening Johnsons, will you please follow me.” We used to use *thou* for singular and *you* for plural (or a more respectful alternative to thou), but it’s died out. And some dialects get around it by using terms like “y’all” or “youse” for the plural, but the fact is, English doesn’t have a lot of options when it comes to singular/plural pronouns!
Because that’s what society has decided on using. We (as a society) toyed around with other options for decades and none of them caught on (see also Xe/Xer/Xis, etc). Finally, a small group of people decided they wanted to use “they” and they did and it caught on, and now “they” is both singular and plural as context demands
It’s cause you’re not used to it. There’s plenty of instances where correct grammatical sentence structure in English still leaves room for ambiguity, but we’ve learned to accept it, aside from the times when we make fun of English.
The use of “they” as a gender neutral term in this fashion has been in use for at least a couple of centuries, but different schools of thought are fixated on their own personal tradition of speaking.
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