GOING TO vs. WILL
GOING TO IS USED
**when you have decided to do something, it's your intention to do it
Susan is going to sell her car. (= it's her plan)
What are you going to wear to the ball tonight? (= what is your plan?)
**when it is clear now that it is sure to happen
It's going to snow. (- you look at the sky and you can see it)
She's going to fail the exam. (- you know that she hasn't studied much and the exam is difficult)
GOING TO IS NOT USED
**when you have just decided to do something – spontaneously
Wait. I'll help you. (= I haven't planned it before)
I'll phone you on Tuesday, OK? (= I have decided)
GOING TO or WILL?
QUICK RULE
GOING TO is used to express a future intention thought about before the moment of speaking.
WILL is used to express a future intention or decision made at the moment of speaking.