When? bao giờ / khi nào: first for the future, last for the past

Khi nào and bao giờ both mean "when". Where you put them tells the listener whether you mean the future or the past.

At the start, they ask about something still to come: Khi nào em đi Đà Lạt? When are you going to Đà Lạt? At the end, they usually ask about something that has already happened: Anh về khi nào? When did you get back? The verb itself never changes, so the position does the work that "are going" and "did" do in English. With a word that points ahead, like định (plan to) or sẽ (will), the end can ask about the future too: Anh định về khi nào? When are you planning to go back?

In Saigon khi nào is the everyday word, and in Hà Nội bao giờ is more common. Both are understood everywhere and follow the same rule. In casual Saigon speech you'll also hear hồi nào for the past and chừng nào for the future. Lúc nào, "at what moment", works the same way.

The answer is a time word, usually in the same place: Tuần sau em đi. Next week. Anh về hôm qua. Yesterday.

Not every khi nào or bao giờ asks a question: Khi nào rảnh thì gọi anh nha. Call me when you're free. And chưa bao giờ means never: Em chưa bao giờ đi Đà Lạt. I've never been to Đà Lạt.

For a clock time, ask mấy giờ. For how long something lasts, ask bao lâu.