This is the 30th most frequent Vietnamese word.
sau
"After," "behind," or "later."
In this sentence, 'sau' is used as a preposition, meaning 'behind' in English, to denote the relative spatial position of the students to the teacher.
Học sinh ngồi sau giáo viên.
The students sit behind the teacher.
Here, 'sau' functions as an adverb, meaning 'later,' and indicates a time in the future.
Chúng ta sẽ gặp nhau sau.
We will meet later.
In this instance, 'sau' is part of the conjunction 'sau khi,' which means 'after,' and it is used to describe the temporal sequence of actions, i.e., watching the movie followed by his arrival.