CAN is used to talk about the ability to do something at a given time in the present or future
Example:
He can speak Chinese fluently.
COULD is used to talk about past time
Example:
I could run 10 miles when I was young.
COULD is used as a polite way of telling or asking someone to do something
Example:
Could you lock the door when you leave?
Can is more informal
Example:
Can you take me to the Victoria station?
CAN is used to make general statements about what is possible
Example:
In Norway, temperatures can be low as -50°C in winter.
COULD is used to show that something is/was possible now or at specific event in the past
Example:
The weather station said temperaturescould be as low as -10°C tonight.
CAN is used to ask for permission to do something in informal way
Example:
Can I borrow your car tomorrow?
Could is more formal and polite than can
Example:
Could I choose my own doctor?
We use can to talk about things which are allowed in the present or future
Example:
You can take a holiday tomorrow if you want.
We use could to talk about things which are allowed in the past
Example:
When I first started this job we couldtake holiday when we wanted.
