| Will + infinitive | Be going to + infinitive |
A decision at the moment of speaking: Marie: We ran out of bread. | A decision before the moment of speaking: Marie: We ran out of bread. |
A prediction based on opinion: I think the Blue Team will win the game. | A prediction based on something we can see (or hear) now: The Blue Team are going to win the game. They already have most of the points. |
A future fact: The sun will rise tomorrow. | |
For promises / requests / refusals / offers: I will help you tomorrow, if you like. |
More examples:
- (The phone rings)
Ben: I‘ll get it!
(‘I’m going to get it’ is very strange, because it makes us think that Ben knew the phone was going to ring before it did).
- I‘m going to go on trip next week.
(‘I’ll go on trip next week’ makes it sound like you’ve only just decided at that minute. Of course, this is possible, but normally we plan our trip more in advance!).