If Erik phones, I'll let you know.
If Erik phones, I'm going to let you know.
Is there any difference in meaning? My grammar book says it does, but the explanation is confusing.
However, we use will, not be going to, when the main clause refers to offers, requests, promises, etc. and ability:
If Erik phones, I'll let you know. (=an offer; '..., I'm going to let you know' suggests 'I intend to let you know when Erik phones')
- Isn't it a promise, not an offer?
- I see no difference in 'I'll let you know' and 'I intend to let you know'.
Another example sentence goes like this:
If you look to your left, you'll see the lake. (=you'll be able to see; '... you're going to see... suggests 'I know this is what you can see when you look to your left').
This is not clear as well.
Please help me figure this out.