Learn Hardcore Vietnamese: Bạn đã xem phim đó chưa?

đã is an aspect marker indicating that the action is completed or in the past. Vietnamese verbs don’t change form for tense; instead you add đã (past), đang (ongoing), sẽ (future), etc.
Example:

Bạn đã xem phim đó chưa? (“Have you watched that movie yet?”)

What role does chưa play at the end of the sentence?

chưa in questions asks if something has not happened yet. Placed at the end, it turns a statement into a yes–no question about completion.

Bạn đã xem phim đó = “You have watched that movie.”
Bạn đã xem phim đó chưa? = “Have you watched that movie yet?”

Why is the order đã xem and not xem đã?

In Vietnamese, aspect markers like đã always come before the verb they modify. Putting đã after the verb would be ungrammatical.

Can I omit đã and just say Bạn xem phim đó chưa?

Yes. Dropping đã is common in informal speech.

Bạn xem phim đó chưa? still means “Have you seen that movie?”
The past sense is implied by at the end.