If … then: nếu … (thì)

Nếu means "if". Put the condition after nếu, then the result, usually starting with thì: Nếu mai trời mưa thì mình ở nhà. If it rains tomorrow, we'll stay home. Thì works like "then": it marks where the result begins. It's optional, but very common in speech.

The result can also come first, and then there's no thì: Gọi chị nếu em cần gì nha. Call me if you need anything.

In speech you'll often hear nếu mà, which means the same and sounds more conversational: Nếu mà kẹt xe thì em tới trễ. In relaxed talk nếu often drops out, leaving just thì: Rảnh thì ghé nhà chị chơi nha. Drop by if you're free. The verb never changes, and sẽ (will) is usually not needed.

English "if" can also mean "whether": "I don't know if Minh is coming." Nếu can't do that. Vietnamese puts có … không inside the sentence instead: Tôi không biết Minh có tới không. And for something that will surely happen, use khi (when) rather than nếu.