forms of can
can + verb
I can swim quite well.
Cannot + verb
We can not afford a new car.
Can you speak any other languages?
We use can and can not for the present and the future.
We use could and could not for the past.
We use can and can not to talk or ask about the ability to do something , in general or at a particular moment in time .
We use can / can not or will/ will not be able to to say that something will/will not be possible in the future .
We use could and could not to talk about past abilities in general.
We usually use was/were able to or managed to (not could) to talk about being able to do something successfully at a particular time in the past. this might be to describe something that was difficult to do .
EX: After several days , the climbers were able to get/ managed to get to the top of the mountain.
We use could not to say that something was impossible for someone in the past .
EX: he was listening to music on headphones and he could not hear me.
We use can , could , may , and would in questions:
Can I help you?
Could I borrow your pen, please?
May I ask a question?
Would you wait for a moment , please?
We use can / can not and be allowed to with the same meaning:
You can park here./ You are allowed to park here.
I Can not use my fathers computer./ I am not allowed to use my fathers computer.
We use Can I ? to offer to do something for someone:
Can I give you a lift?
We use Can I /We ? Could I/We? and May I ? to ask for something or for permission to do something.
Can I have a go now?
Can I ? is not as polite as Could I ? May I ? is very polite and is used in formal situations . Here , someone is talking to an interviewer in a job interview:
Could/May I ask a question?
We use can you? could you? and would you?to ask someone to do something . Can you? is not as polite as Could you? and Would you? Here , someone is talking to a friend:
Can you be quiet?
Here someone is talking to a person they do not know.
Could/Would you be quiet, please?
We use can and can not to give or refuse permission to someone:
Yes, you can use my phone.
You can not sit there, it is my friends sit.
We use can / can not and be allowed to to talk about what is permitted or not permitted by rules and laws:
You can go to this club if you are over 18.
You are not allowed to go to this club if you are under 18.