|it is simple:
use it to talk about a fact in the future,
something that is certain
|The sun will rise at 7 am.
She'll leave in the afternoon.
They'll arrive in the evening.
as a sort of promise
(I want to do that)
|I'll help you with your homework
It is cold. I'll close the window.
|for a future prediction (what you expect to happen then)
you may use simple future
or going to
It will rain tomorrow.
It's going to rain tomorrow
In many sentences there are clues that tell you:
tomorrow, next week (month, year...),