'I couldn't get up at 4:00 tomorrow.' is this sentence wrong?

Question

Today, I(non native speaker) read a question on a Q&A in my nation. The question is 'what is difference between A and B?'

A. I can't get up at 4:00 tomorrow.
B. I couldn't get up at 4:00 tomorrow.

everybody(we are non native speaker) said 'could' is the past form of 'can' so you can't use "couldn't" with tomorrow. Because 'tomorrow' means future tense not past tense. so B is wrong in grammar.

Of course 'could' is the past form of 'can' but also 'could' includes 'possibility' so I think B is possible.

I wonder B is wrong and absurd? (especially in grammar)

Answer 1

The second case is good English in a subjunctive mood. Consider this exchange:

Can you pick me up at six? No, I can't get there until eight.

This is indicative mood: it expresses an expected future, not merely a possible one.

I could be there by six if traffic weren't so bad!

The positive subjunctive is a little more common. I think negative subjunctive is most likely a response or amendment to a previously expressed or thought positive subjunctive.

No, wait, I still couldn't be there by six even if the roads were empty!

By the way, this negative subjunctive future was recently used in the TV show The Last Man on Earth when the title character said,

have you ever heard the phrase, "if you were the last person on earth, I still would not have sex with you!"?

It's particularly funny because in the story the characters believe they actually are the last people on earth, so the indicative mood would have been appropriate if it hadn't been a quote.

Answer 2

Option A definitely sounds better. However, if you get rid of the negative, both "I can get up at 4 tomorrow" and "I could get up at 4 tomorrow" sound equally fine (the first more referencing your ability to get up, and the second more referring to the general possibility").

If you flip the sentence to talking about yesterday, option B is the better choice - "I couldn't get up at 4 yesterday", not "I can't get up at 4 yesterday". Getting rid of the negative makes both sentences sound bad, though.