In this dictionary, the modal verb "would" has 14 meanings.
Ok, let see this very short conversation:
A: I'm going to the cinema tonight. Do you want to come along?
B: Yes, I do.
A: That would be very interesting / That will be very interesting.
Ok, let see the sentence "That would be very interesting". According to the above dictionary, that sentence should belong to a conditional sentence type 2 "That would be very interesting if you came along". The conditional sentence type 2 expresses something that is not possible to happen at the present time or in the future.
So, if A say "That would be very interesting", it could be that he thinks that it is impossible for him to come along.
Ok, let see the sentence "That will be very interesting". According to the above dictionary, that sentence should belong to a conditional sentence type 1 "That will be very interesting if you come along". The conditional sentence type i expresses something that is possible to happen at the present time or in the future.
So, if A say "That will be very interesting", it could be that he thinks that it is possible for him to come along.
So, which one to choose "I will be very interesting" or "It would be very interesting"?
I think "It would be very interesting" is more polite, but I don't know why?.
So, conditional sentence type 2 is more polite than conditional sentence type 1?
Best Answer
First, understand it's somewhat weird to have a conversation where A asks B something, B accepts, then A immediately questions it. So this means there are things going on that are not being explicitly talked about.
There are many reasons in language to "indirect-ify" things - examples include telling superiors something sensitive, simply being polite, business dealings, taboo subjects, trying to get someone to take a hint, etc.
Modals, particularly would, should, could, often are involved in these situations where you want to say something, but can't be direct due to some social propriety, either real (in the case of formal meetings, etc.) or fake for humorous/sarcastic effect. Used wrong, modals can make you sound snobbish.
So, your example:
What I gather from this conversation is A did not take B's acceptance seriously at all. It's an indirect refusal. It might even be A snubbing B - maybe A didn't really want to invite B, and B expected A to politely refuse. This is a type of conversation that would probably make much more sense spoken where you can gauge the mood and emotion involved a lot better. I can imagine A laughing at B's acceptance, and then saying "That would be very interesting."
Now, this:
Here, something will happen if B goes to the cinema, and A knows it. A was not entirely sure if B would go and believed that he likely would not. Perhaps B's ex-girlfriend is going to be there. Or someone else that A knows B wants to avoid. However, there is no snobbishness here. A is simply making a comment about what B wants to do.
Hopefully that makes sense. Would is definitely more polite in most situations but it can be used to opposite effect as well.