When: khi … (thì)

Khi means "when" and joins two sentences: Khi rảnh, tôi hay đi bơi. When I'm free, I often go swimming. The khi part usually comes first, then what happens.

When the khi part comes first, the second part often opens with thì, a small "then": Khi em tới thì chị Lan về rồi. When I got there, Lan had already gone home. Thì is common in speech and can be left out. If the khi part comes second, there's no thì: Tôi không thích ra đường khi trời mưa.

In Saigon conversation, lúc, which you know from clock times, often takes the place of khi: Lúc mới qua Việt Nam, tôi không biết nói gì hết. Khi is the usual word in writing.

The verb stays the same for past, present and future; the rest of the sentence shows when, with words like rồi, sẽ or a time word.

Khi on its own doesn't ask anything. To ask "when?", use khi nào or bao giờ: Khi nào em về? For two things going on at once, "while", use trong khi.