I'm reading an article about two pets rescuing their owners. After the cat managed to wake up the mother, she tried to wake up other members but they wouldn't wake up.
Cathy tried to tell her husband and son to leave the house, but they wouldn't wake up.
Why wouldn't and not didn't?
(I checked possible duplicates but they weren't convincing at all. Besides, none of the answers were confirmed. One said would is used for permission and another said it's for emphasis!)
Wouldn't, in such sentences, means roughly "couldn't be caused to"; that is, "X wouldn't Y" means roughly "[someone] couldn't get X to Y". For example:
Cathy tried to tell her husband and son to leave the house, but they wouldn't wake up.
≈ Cathy tried to tell her husband and son to leave the house, but she couldn't wake them up.I knelt on the seat and tried to pull up on the window, but it wouldn't budge. [link]
≈ I knelt on the seat and tried to pull up on the window, but I couldn't budge it.She wouldn't give me a straight answer on whether or not she was going to pay the rent and she wouldn't give me the keys; she just left. [link]
≈ I couldn't get a straight answer on whether or not she was going to pay the rent, and I couldn't get her to give me the keys; she just left.
Likewise, won't means roughly "can't be caused to":
I try to will my voice from shaking, but it won't stop. [link]
≈ I try to will my voice from shaking, but I can't get it to stop.
Both are grammatically correct, but there is a slight difference in meaning.
As Yuri mentioned, there's a slew of Stack Exchange posts on the topic (e.g., 1, 2, 3) but even English teachers get confused about this. This BBC post for "Ask About English" is rather straightforward. The upshot is:
Cathy tried to tell her husband and son to leave the house, but they didn't wake up.
Both verbs are in the simple past tense. Cathy tried to do something. Her son and husband did not do something. One thing happened, then another didn't happen, and it was all sometime before now. That's it.
Cathy tried to tell her husband and son to leave the house, but they wouldn't wake up.
Would is both the past tense and the subjunctive mood of will, for which the OED offers 52 primary senses. There's a horde of meanings, some more odd and obscure than others.
The original and primary sense is to want, desire, choose. Here, that would mean that she attempted to speak to them but they opted or chose not to wake, in the manner of a kid half-consciously banging a cheap alarm clock when it goes off at 7 am on Saturday morning. This doesn't seem to fit here: they aren't seemingly conscious enough to be making any decisions.
By extension, it came to mean to habitually want, desire, choose and therefore to have the habit of. Here, that would mean that she attempted to speak to them but, as was their habit, they remained sleeping. That seems closer, but it involves knowledge of their usual habits or a judgment about their personalities... neither of which seem to show up.
By extension, it came to mean to be able to, to be capable of, to have the potential to and—without going into the rest of the word's senses—that's what's going on here. She tried to wake them but not only did they not wake up: they were incapable of being awoken. Since she and we would only know that if attempts were made to awaken them, @Qvalador is right that it implies there was some effort involved in a way that didn't doesn't.
I think it's about subtle nuances. Both wouldn't and didn't make sense.
While didn't simply states that they didn't wake up, using wouldn't feels like,
They didn't wake up, and plus it seemed they wouldn't wake up even if Cathy tried harder.
My PC doesn't work, My PC won't work. The first sentence means that the PC doesn't work, and the second one means that it doesn't work, and it seems it's not going to work however hard I try to make it work.
The alarm rang, but I didn't wake up - that's a rather normal thing; I will be at work a bit late because I'm going to stay in bed too long.
After she heard a gunshot, Mrs. Smith ran to the bedroom and shook her husband, but he wouldn't wake up - it seems that unfortunately Mr. Smith isn't going to wake up ever, because he has been shot or shot himself.
"Wouldn't" means "it is not going to happen". In the case of "wake up", it implies that something bad happened to the person, because everybody is going to wake up eventually, unless they are dead or in a coma. "She tried to give the guard 50 dollars, but he wouldn't let her pass" - it's not going to happen that he lets her pass, probably because he doesn't accept bribes. "It is not going to happen" is implied by "wouldn't". The exact reason depends on the context.
The difference I clearly see is that wouldn't here may mean unwillingness, inability or impossibility and is just a little less explicit than didn't
According to BBC Learning English (one I trust completely):
They didn't wake up. - Quote: (This is just a simple statement of fact. It’s completely neutral; it’s just giving the information.)
They wouldn't wake up. - Would is a modal verb. This statement carries the following meaning.
Could be wrong here, but as far as i'm concerned the differences here are fairly subtle. The use of wouldn't indicates that they didn't wake up despite attempts to make them, whereas didn't just indicates that they... did not, without any reference to attempts to do so. Wouldn't implies effort; didn't does not.
