What? gì after the verb

Gì means "what". It goes where the answer will go, which is usually after the verb: Trưa nay em ăn gì? What are you having for lunch? The answer swaps it for the thing: Em ăn phở. Anh làm gì? asks what you're doing, or what you do for a living.

To ask what something is, use là gì: Cái này là gì? What's this?

When "what" is the one doing something, or the thing happening, gì can't stand alone. Put cái (thing) in front of it for a thing, or chuyện (matter) for something happening: Cái gì ở trong này? What's in here? Có chuyện gì vậy? What's the matter? Cái gì also works after a verb, where it sounds more emphatic and can come across as sharp: Em nói cái gì?

Straight after a noun, gì asks what kind: món gì? (what dish?), tiếng gì? (what language?). To pick one from a set, use nào: cái nào? which one?

After không, gì means "anything": Em không ăn gì. I'm not eating anything. Không có gì. It's nothing (also the reply to a thank-you: you're welcome).

Vậy at the end makes a question sound softer and friendlier: Anh làm gì vậy? In Hà Nội you'll more often hear thế.