Question words stay where the answer goes: Anh đi đâu?, Ai đi?

In English, the question word jumps to the front: "What does Lan like?" Vietnamese keeps the order of the answer and puts the question word exactly where the answer will go. Chị Lan thích cà phê (Lan likes coffee) becomes Chị Lan thích gì? Chị Lan làm ở ngân hàng (Lan works at a bank) becomes Chị Lan làm ở đâu?

So to answer, take the question and swap the question word for your answer, changing the pronoun to fit: Anh muốn ăn gì? → Em muốn ăn phở.

Ai (who) at the start of a question isn't an exception. When it asks who did something, the doer comes first in the answer too: Ai gọi em? Who called you? Chị Lan gọi em.

A few question words usually do go first. One is tại sao (why), or just sao in everyday speech: Sao em học tiếng Việt? Another is khi nào (when), for something still to come: Khi nào anh về? When are you going back? Put khi nào at the end and it usually asks about the past: Anh về khi nào? When did you get back? With a word for plans, such as định or sẽ, the end works for the future too: Anh định về khi nào? In Hà Nội you'll hear bao giờ more often, in the same two places. Mấy giờ (what time) can go first or last, with no change in time: Mấy giờ anh đi? or Anh đi lúc mấy giờ?

In a negative or a yes/no question, the same words mean "anything" or "anywhere": Em không ăn gì. I don't eat anything. Anh có đi đâu không? Are you going anywhere?