If you have + you will have

[5jj]

But it means sth something is gonna going to happen in the future and it is certain when "will" is used. Right?

Please try to avoid chatlish and unnecessary abbreviations in this forum.

Not always.

It is nearly midnight. Mary will be in bed now.

Then, how do you express that you're going to do sth in the future?
There are several ways:

Emma leaves for Paris tomorrow.
Emma is leaving for Paris tomorrow.
Emma will leave for Paris tomorrow.
Emma is going to leave for Paris tomorrow.
Emma will be leaving for Paris tomorrow.

There is a slight difference between the messages conveyed by these, but they all refer to Emma's future leaving.

5

[5jj]

Maybe you see my point Fivjedjon, when I say teachers have made it seem unnaceptable.

Indeed. I have heard far too many teachers say "Never use will (or would) in an if- clause.