This question already has an answer here:
As we know, when the pronoun someone is used, the succeeding verb will be conjugated in the 3rd-person singular. Thus, the following sentence demonstrates legal usage:
I cannot enter the room; someone is cooking.
However, note the following sentence:
I cannot enter the room; someone is cooking, are they not?
As can be seen in the second sentence, when a question tag is used, it doesn't use the corresponding conjugation of the 3rd-person singular previously used, but rather, it takes on the 3rd-person plural. Why is this the case? What is more, why is the following sentence illegal?
I cannot enter the room; someone are cooking, are they not?
I thank you, in advance, for your responses.