It is used to decide at the moment of speaking. very often it is used to offer something to somebody:
I will open the window.
I will get it.
It is used when you want to agree or promise to do something as well as when you want to ask somebody to do something:
I will give it back to you tomorrow.
I will take care of your garden perfectly. I promise.
Will you please turn the music down?
It is used to talk about the future facts:
She will be 38 next month.
Christmas will be cold.
It is used to say what you think or hope will happen:
I think I will pass the exam.
They hope everything will be ok.
In my opinion, she will win the fight.
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ATTENTION!
SHALL I/SHALL WE
THIS FORM IS A BIT OUTDATED BUT YOU CAN STILL HEAR IT BEING USED.
SHALL IS USUALLY USED IN QUESTIONS WITH PRONOUNS I AND WE TO ASK SOMEBODY'S OPINION.
SHALL I TALK TO HER?
SHALL WE GO TO THAT RESTAURANT?