"Could" is a modal verb used to express possibility or past ability as well as to make suggestions and requests. "Could" is also commonly used in conditional sentences as the conditional form of "can." Examples: Extreme rain could cause the river to flood the city.

When we use can and could?

Can, like could and would, is used to ask a polite question, but can is only used to ask permission to do or say something ("Can I borrow your car?" "Can I get you something to drink?"). Could is the past tense of can, but it also has uses apart from that--and that is where the confusion lies.

When should and could is used?

Just remember that could is used to talk about something that can happen, would is used to talk about something that will happen in an imagined situation, and should is used to talk about something that ought to happen or must happen.