1. When to use WILL
- Express future actions decided at the moment of speaking (Immediate Decisions)
I'll have salad now.
- Express a prediction based on personal opinions or experiences (Predictions without Evidence)
I think United will win the game.
- A future fact
The sun will rise tomorrow.
- To make a promise, an offer, a threat or refusal
I promise I won't tell anyone you broke the window.
I'll take you to the airport tomorrow.
I'll tell your parents what you did.
No, I won't cook your dinner, you can cook it yourself.
2. When to use BE GOING TO
- Express future plans decided before the moment of speaking (Prior Plans)
I'm going to visit my aunt next Friday.
- Express a prediction based on present evidence (Predictions with Evidence)
Look at those black clouds. It is going to rain.
- Something is about to happen
Get back! The bomb is going to explode.
Both Will and Be Going to can be used for making future predictions without having a real difference in meaning
I think it will be foggy tomorrow. = I think it is going to be foggy tomorrow.