Future: Will vs. Be Going To

Future with WILL

I'LL speak
(short for I WILL).
Negative: I WON'T speak (short for WILL NOT).
Question: WILL you speak?

Will is a modal verb. It's used for actions you decide you'll take AT THE MOMENT (as a reaction during a conversation, meeting, making plans), actions that are NOT CERTAIN TO HAPPEN, or PROMISES (in this case WILL is not shortened as 'LL).

Examples of Future with WILL

You look tired. I'LL bring you a coffee (you're deciding it NOW).
I think I WON'T pass the exam (but you're NOT SURE).
Ok! I WILL stop calling you (a PROMISE).

Future with BE GOING TO

I'M GOING TO speak.
Negative: I'M not going to speak.
Question: ARE you going to speak?

See how the only auxiliary verb is the verb TO BE, so GOING TO never changes its position. It's used when an action is ALREADY planned, so you are just REPORTING it.

Examples of Future with BE GOING TO

I'M GOING TO drop these documents at the tax office (it's sure and I'm just TELLING you).
We'RE GOING TO hold the meeting at the Main Boardroom (for sure).
She'S GOING TO study an MBA in Harvard next year (sure).

And now practice with this exercise on Future with Will and Be Going To