While: trong khi, and khi … đang for an interrupted action

Trong khi means "while": two things going on at the same time. Trong khi mẹ nấu cơm, ba dọn bàn. While Mum cooks, Dad sets the table. Like khi, the trong khi part usually comes first, and it can also follow: Tôi đọc sách trong khi chờ xe buýt. In conversation, trong lúc is just as common.

For one action that cuts into another, put đang (in the middle of) before the longer one: Khi tôi đang ăn cơm thì có người gõ cửa. While I was eating, someone knocked at the door. The đang action was going on; the other one interrupted it. Khi, trong khi or, in speech, lúc can start it, and thì often opens the second part.

Trong alone goes before a noun, "during": trong bữa ăn (during the meal), trong giờ học (during class). Before a whole sentence it needs khi or lúc.

English "while" can also set two facts against each other: "I like tea, while my wife likes coffee." In conversation, Vietnamese uses còn for that: Tôi thích trà, còn vợ tôi thích cà phê. In writing, trong khi and trong khi đó do it too.