could vs. can

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.


Why does Morty use couldn’t instead of can’t? What’s the difference?


Couldn’t is past tense. Can’t indicates present tense.

In this example, either one may be used without materially changing the meanimg.

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 'ifthen 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.