Some grammar purists insist that they should only be a plural pronoun, but its use as a singular pronoun is steadily increasing,
There are several situations where they is used as a gender-neutral singular pronoun.
1. As a replacement for he or she
It has become both politically correct and lexicographically easier to write they rather than he or she, he/she, (s)he or s/he. As an example “After the user clicks the button, he or she must enter his or her name” is rather written “After the user clicks the button, they must enter their name”
The gender neutral alternative would be it, but it is very strange to write “After the user clicks the button, it must enter its name”. This solution could make sense but it is never used in reference to human beings.
In France and many other countries, the masculine form is used in such cases, but it has been criticized.
I’ve also seen authors using seemingly randomly masculine or feminine forms of the pronoun to avoid the appearance of politically incorrectness.
As these solutions are inconsistent, using the they pronoun makes sense and it is becoming standard usage.
2. With someone, everyone or nobody
There are many examples of such sentences:
– Everybody should choose their seat.
– Someone has done their homework.
– Nobody emptied their plate.
Actually, again if we insist on using a “true singular pronoun”, one and one’s could replace this usage.
– Everybody should choose one’s seat.
– Someone has done one’s homework.
– Nobody emptied one’s plate.
But here again, this sounds weird, and their has become standard usage.
3. As a designation for transgender or “gender-fluid” people
Many transgender people don’t like to be referred as he or she (see our article on the use of Mx instead of Mr/Mrs/Ms in British English), so they has been used by some to solve this issue. I admit that I find this use strange, particularly for transgender people, whom I thought would rather like to be addressed according to their current gender.
In any case, those who insist that the use of the “singular they” is ungrammatical are really swimming against the linguistic current!