The word, "can" has virtually entirely supplanted the word, "may", in English Language usage in the United States of America. People (and scriptwriters) use "can" when they should use "may". This is significant.
"Can" refers to ability.
"May" refers to possibility and also permission.
By failing to make the distinction, linguistically, people subliminally confuse ability with permission. The general trend is to shift responsibility away.
"Can I go to the bathroom?" is a common confusion. Hopefully, the answer is, "yes,". Even people with prostate problems or constipation rarely raise the question, "Can I go to the bathroom?". They just go and hope for the best.
What's really meant, however, is, "May I go to the bathroom?" This is a different question than, "Can I go to the bathroom?". It asks permission of someone else. It's a totally different communication.
Another confusion of "could" is with "would" ,
As in, "Could you come here?" The word, "could", leads to a similar confusion, to which an understandable answer might be, "Wha'?" But people actually say that. "Could you come here?"
Of course, in the situation in which someone might say that, the answer would generally be, "Yes," with a momentary flicker of doubt as to the competence of the person.
What they mean is, "Would you come here?". That shifts the meaning from ability to willingness. Because -- you never know.
But they say, "Could you come here?" instead. Why?
Ability vs. willingness
Can't vs. won't
Could vs. would
It has to do with where responsibility lies. "Can't" may be excused. "Won't" assumes responsibility for the consequences.
But some people say, "Can't", when what is true is, "Won't". They won't. But they say that they can't, as in, "I'm sorry, I can't do that."
It follows that such people are avoiding responsibility for the consequences of, "Won't". They are assuming a childlike non-responsibility ("can't"). When a child "won't", you know it; and you know when a child "can't". Same is true of adults.
AND we're back to
"Can I go to the bathroom?"
"I'm SO glad you asked. How is your health?"
"May I go to the bathroom?"
(an intelligent question, especially under the circumstances)
You may . . . . .
You may not.
can and may
could and would
can't and won't
Catch my drift???