Who? ai as subject or object

Ai means "who". Like most question words, it stands where the answer will go. To ask who did something, ai comes first, because the person who did it comes first in the answer too: Ai gọi em? Who called you? Chị Lan gọi em. Lan called me.

To ask who something is done to, ai goes after the verb: Em chờ ai? Who are you waiting for? Em chờ ba. I'm waiting for my dad. After với (with) or cho (for), it goes at the end in the same way: Em đi với ai? Who are you going with?

To ask who a person is, put là ai after them: Người đó là ai? Who is that? Turn it round, Ai là …?, and you ask which of several people it is: Ai là anh Minh? Which one of you is Minh? You ask that when you know the name but not the face.

For "whose", put của ai after the thing: Cái này của ai? Whose is this?

Not every ai asks a question. In a statement it means "anyone" or "everyone": Ai cũng biết. Everyone knows. Không ai biết. Nobody knows.