Compare will and would:
- I'll stay a little longer. I've got plenty of time.
I'd stay a little longer, but I really have to go now. (so I can't stay longer)
- I'll call Lisa. I have her number.
I'd call Lisa, but I don't have her number. (so I can't call her)
(Taken from English grammar book)
From what I've understood after reading the examples above, can I conclude that will is used to express desire in the future spontaneously (my book told me to use "be going to" when the event is planned), but wouldn't is used to say something that failed to happen (wouldn't happen)? Is that correct?
By the way, these are some references that I've read, but got no explicit information about what I'm asking.
Most explanations said would is just the past form of will or in another case, would is softer than will when it's used to ask someone to do a favor or to offer something to someone. (Would you like..., would you please..., etc).
In my case, there's no unambiguous clause using the past verbs, so why do we use "would"?
To make sure that I have understood, here I made some sentences and let me know whether they're correct or not. Thanks.
- I'll buy some books tonight.
I'd buy some books, but I don't have money. (I won't be able to buy some books)
- I'll paint the wall because there's a stain on it.
I'd paint the wall, but I don't have paint. (I won't be able to paint the wall)