Will + infinitive
Will has a neutral meaning. We use it to talk about facts in the future:
- I’ll be 31 next month.
- Next year will be 2018.
- She’ll finish her study next year.
- The sun will rise tomorrow.
We use will for instant decisions or agreement at the moment of speaking:
- – There’s no milk. – Really? In that case, I’ll go to get some.
- There’s a postbox over there. I’ll post these letters.
We use will for prediction based on opinion:
- I think the Conservatives will win the next election.
We use will for a prediction about the future:
- One day people will travel to Mars.
We use will in offer/invitation/promise:
- I’ll help you tomorrow. if you want. /offer/
- Will you come to lunch? ~Yes, thank you. /invitation/
- I’ll pay you back next week. /promise/
Will does not express an intention /we use going to/:
- It’s her birthday. She’s going to have a meal with her friends. NOT
She’ll have a meal.
But we often use be going to for an intention and will for the details and comments:
- We are going to have a meal. There’ll be about five of us. Oh, that will be great.
Be going to + infinitive
We use be going to for an intention, something already decided to do before the moment of speaking:
- I’m going to sell my car.
- He’s going to quit his job next month.
- I’m going out. I’m going to post these letters.
We use be going to for a prediction based on something we can see /or hear/ now:
- There isn’t a cloud in the sky. It’s going to be a lovely day.
- This bag isn’t very strong. It’s going to break.