Morty and Jerry walk into a gym. They see a old man is lifting weights.
“You [color=red]couldn’t do that,” says Morty to Jerry.
“I could. I just choose not to,” says Jerry.
Questions:
Why does Morty use couldn’t instead of can’t? What’s the difference?
The use of ‘could’ indicates that the person is referring to a hypothetical situation: “You couldn’t do that (if you tried).”
It’s the usual 'if…then would/could (wouldn’t/couldn’t)
The use of ‘can’t’ in this context would be used if Jerry then tried, struggled hard, and nearly drops a heavy weight on his foot; and Morty says, in good humour, having been proved right:
“Stop. Stop before you injure yourself. You can’t do - admit it.”
Because now, the situation is real - he is actually trying to do it.