Can I use 'they' for a singular person?
It is possible to use 'they' when referring to a singular person.
Here is an example:
"If a person knocks at the door late at night, I get worried because I don't know who they are."
This happens because there is no neutral singular form in English. Sometimes we don't know or don't think it is important if a person is a he or a she, so we say 'they' instead.
However, if you have a doubt, and it makes sense grammatically, you can always change the sentence.
Make it plural:
"If people knock at the door late at night, I get worried because I don't know who they are."
Use 'he or she':
"If a person knocks at the door late at night, I get worried because I don't know who he or she is"
