Saying it is happening now: đang

Đang before a verb says something is going on right now: Em đang nấu cơm. I'm cooking. It's close to English "-ing", but used less: when the situation makes it obvious, people often leave it out.

It also works before an adjective, for a state you're in at the moment: Chị đang bận. I'm busy right now. Vẫn đang means "still": Tôi vẫn đang chờ. I'm still waiting.

Đang isn't only for now. It describes something in progress at any moment you name, often with lúc (when, at the time): Lúc anh gọi, em đang tắm. When you called, I was in the shower. That moment can be in the future too: Giờ này mai tôi đang ngồi trên xe đi Đà Lạt. This time tomorrow I'll be on the bus to Đà Lạt.

The trap is English "I'm going to Đà Lạt tomorrow". That "-ing" is a plan, not something in progress, and đang can't carry it. For a plan, the time word is enough: Mai tôi đi Đà Lạt. Add sẽ if you want to make the future clear. For something already done, use đã.

You'll ask Anh đang làm gì vậy? (What are you doing?) all the time. In Hà Nội it ends in thế instead: Anh đang làm gì thế?