I received a question from a former student in my email:
"How could I explain the difference between the verb 'can' and 'could'? Does 'could' involve with one's willingness to do something and does 'can' talk about one's ability?".
I then replied:
"I can understand your confusion. there is more than one meaning for 'can' and 'could".
'Can' generally refers to ability (to be able to do something).
"I can walk in twelve minutes".
"I can speak Japanese".
'Could' however refers to past ability.
"When I was young, I could walk a mile in 8 minutes".
"At one time, I could speak Japanese. But then, I have forgotten how".
Sometimes, both 'can' and 'could' are used to make a polite request.
"Can you help me, please?".
"Could you help me, please?"