Study materials
The phone rings. You had no plan to answer it, nobody asked you to, the decision appeared right now, in half a second.
That is what will is for. It is a future that did not exist a moment ago.
The phone is ringing. I'll answer it.
The form is the simplest of all the futures: will plus the base verb, the same for every person. No am, is or are, and no s.
A decision made this second
The most typical case is a reaction to something that has just happened or has just been said.
It's cold in here. I'll close the window.
We're out of milk. I'll buy some.
Promises and offers
Two more uses grow from the same root. A promise is a decision about the future that you say out loud. An offer is a decision to help.
I'll call you tomorrow, I promise.
I'll carry that bag for you.
Don't worry, I'll help.
A prediction you think rather than see
The third use is a prediction resting on your opinion or experience. The words that give the opinion away are almost always next to it: I think, probably, perhaps, I'm sure.
I think the team will win.
She'll probably be late.
I'm sure you'll like it.
Will or be going to
Here is the question actually worth attention. Both structures point at the future, but they look at it from different sides.
Will goes where the decision or the opinion appeared just now. Be going to goes where it existed earlier, or where the evidence is visible.
We're out of milk. I'll buy some.
I'm going to buy milk after work.
It will be cold tomorrow.
Look at those clouds. It's going to rain.
Won't and questions
The negative is will not, which in speech is nearly always won't. In a question, will simply moves in front of the subject.
She won't forget.
Will they come?
When will you know?
The short answer repeats will itself, as it does with every other helper.
Yes, I will. No, I won't.
The mistakes everyone makes
The first is putting to after will.
I will to call you. → I will call you.
The second is adding an s in the third person.
She will calls you. → She will call you.
The third is reaching for be going to when the decision was made this second.
The phone is ringing. I'm going to answer it. → I'll answer it.
All three are caught by one question. Did the decision appear just now, or did it exist before? Just now is will; before is be going to.
Try it right now
Listen to yourself: what do you feel like doing this minute? Say it in English with will.
Then remember something you planned yesterday and say that with be going to. The gap between those two sentences is the whole topic.