*The uses of can:
1- Ability(am,is,are able to):
EX:I can speak three languages fluently.
EX:I am able to speak three languages fluently.
2-Strong possibility:
Here ,can indicates that something is highly capable of happening as a certain result of another action.
EX:Don't light a match in this chemical factory!
The fire can cause an explosion.
3-Suggestion:
When you don't understand a problem or any thing,I give you an idea or suggestoion as follows:
EX:You can ask Samuel to help you.He is an excellent teacher.
4-Permission:
Can is used to ask for and to give permission:
EX:Can I borrow your camera for one hour?
-Sure.You can.
EX:Can I use your pen,please?
-Yes ,you can.
*The uses of could:
1-Ability in the past(was.were able to):
-Here ,could is used with a past adverbial phrase or clause:
EX:I could play tennis three years ago.
2- Ability based on certain conditions:
EX:We could leave now if we want to.
EX:I could be a good chess player if i want to.But i am not interested.
3-Strong Possibility:
EX:Don't drive so fast!We could have an accident.
4-Permission:
-Could is more polite than can:
EX:Could I borrow your camera?
Ex:Could I use your pen,please?
5-Polite Request:
EX:Could you open the door,please?
EX:Could you fetch that chair?
Note: there is a big difference between permission and request.When asking for permission,the speaker wants to do something.But when making a request.the speaker wants someone else to do something.
6-Suggestion:
This use is the same as suggestion with can:
EX:Someone doesn't have any money.I suggest a follows
-You could call your father and ask him for some money.
Or you could borrow some from your friend.
Note: (Could+have +p.p) is used to express ability that wasn't used in the past .
EX:You could have asked your friends about the appointment.But you didn't.