http://www.englisch-hilfen.de/en/grammar/hilfsverben2.htm

Modals – English Grammar

1) can

Use Examples
ability to do sth. in the present (substitute form: to be able to) I can speak English.
permission to do sth. in the present (substitute form: to be allowed to) Can I go to the cinema?
request Can you wait a moment, please?
offer I can lend you my car till tomorrow.
suggestion Can we visit Grandma at the weekend?
possibility It can get very hot in Arizona.

2) could

Use Examples
ability to do sth. in the past (substitute form: to be able to) I could speak English.
permission to do sth. in the past (substitute form: to be allowed to) I could go to the cinema.
polite question * Could I go to the cinema, please?
polite request * Could you wait a moment, please?
polite offer * I could lend you my car till tomorrow.
polite suggestion * Could we visit Grandma at the weekend?
possibility * It could get very hot in Montana.