1) Use can / can’t to talk about your abilities now.
I can speak English.        I can’t speak German.

    Use could / couldn’t to talk about abilities in the past.
I could speak French when I was a child, but I can't now.
I couldn’t speak English when I was a child, but I can now.

2) Use can and could to make requests.  Could is more polite.
    Can you cook this evening please?           
    Could you pass me the salt?

    Use can to reply to requests.
    Can I sit here?                   =>           Yes, you can.      Sorry, you can’t.
    Can you cook this evening please?           =>           Yes, I can.            Sorry, I can’t.

    DON’T use could in replies to requests.
    Could you lend me some money?
    Yes, I could.          =>           Yes, sure!  Sorry, I can’t.         

1) Can and could are the same for all persons.
I         can/could            speak English
you can/could            speak English
he / she / it         can/could            speak English 
we    can/could            speak English 
they            can/could            speak English.
2) The negative form of can is cannot, or can’t. The negative form of could iscouldn’t.

3) There is always a verb after can and could, and the verb is always in theinfinitive form (without to).
     Sally can help you.     NOT Sally can helps you. / Sally can to help you.

4) Make questions by inverting can and the subject.
     I can see you this afternoon.       =>    Can I see you this afternoon? 
     You could help me.    =>    Could you help me?        
5) Use can/can’t and could/couldn’t in short answers.
     Can your brother swim?         =>    Yes, he can.  No, he can’t. 
     Could you do the test?        =>    Yes, I could.  No, I couldn’t.