Compare will and be going to:
1. We use will with an infinitive (do, go, be, arrive etc.):
will + infinitive
- John will arrive (tomorrow).
|I/you/he (etc.) will/'ll go.||I/you/he (etc.) will not/won't go.||Will I/you/he (etc.) go?|
We use be going with to with an infinitive (to do, to be, to rain etc.):
gong to + infinitive
- It's going to rain (soon).
|My friends are going to come tonight.||It isn't going to rain today.||What are you going to do on Sunday?|
2. We use will for actions that we decide to do NOW, at the moment of speaking:
- I like this coat. I think I'll buy it.
- A: What would you like to eat?
B: I'll have a pizza, please.
We can use will for offers and promises:
- I'll carry your case for you (offer).
- I won't forget your birthday again (promise).
We use be going to for actions that we have decided to do BEFORE we speak:
- I'm going to clean my room this afternoon (I decided to clean it this morning).
We can ask questions about people's plans:
- Which train are you going to catch (which train have you decided to catch)?
3. We use will to talk about things that we think or believe will happen in the future:
- I'm sure you'll enjoy the film.
- I'm sure it won't rain tomorrow. It'll be another beautiful, sunny day.
We use be going to for something that we expect to happen, because the situation now indicates that it is going to happen:
- He's running towards the goal, and he's going to score.