Why? tại sao / vì sao / sao first

To ask why, put tại sao at the start of the question: Tại sao anh thích Sài Gòn? Why do you like Saigon? It's one of the few question words that go first, as in English.

In everyday speech it's usually shortened to sao: Sao hôm qua em nghỉ học? Why weren't you in class yesterday? Vậy at the end makes it more conversational, usually softer, though said sharply it can sound like a complaint: Sao em về sớm vậy? Vì sao means the same and is heard more in the news and in writing.

On its own, Sao vậy? means "What's wrong?" or "Why's that?" (in Hà Nội, Sao thế?).

Answer with vì or, in everyday Saigon speech, tại vì (because); tại on its own often puts the blame on something: Tại kẹt xe. Because of the traffic (in Hà Nội, tắc đường). Often the answer skips the word for because: Dạ, em bị bệnh. I was sick.

Watch where sao goes. After a verb at the end, it means "how", not "why": Anh thấy sao? What do you think? After a negative, a final sao shows surprise: Anh không biết sao? You didn't know? To ask what something is for, use để làm gì.