What is the difference between the following:
- Sorry, mate. I wouldn't be able to come for dinner.
- Sorry, mate. I won't be able to come for dinner.
In most cases, (2) would be used to say that the speaker was unable to accept the dinner invitation. (1) is unlikely to be found on its own. It needs some kind of explanatory setting, such as ‘Yeh, it would be great if we could get together some time. I wouldn't be able to come for dinner, though, because we have a problem getting a baby-sitter.'
Would is often used in hypothetical situations where you wish something to be the case, but in the current situation it is not going to happen.
I would very much like to come for dinner, but I have other plans this evening.
Will is used for things that are actually going to happen.
I will come for dinner.