Hello, Can Is used to talk about possibilities. e.g. 'Can you meet me at 5?' Is used to talk about abilities. e.g. 'I can ride a bike.' Is used to make informal requests and orders. e.g. 'Can you bring me my jumper?' Could Is used to talk about past abilities. e.g. 'When I was a child I could do handstands.' Is used to make formal requests. e.g. 'Could you fax me that document by Tuesday?' Be able to Is sometimes used instead of can or could.  Can be used with all tenses. e.g. 'I will be able to see you next week.' Is used to talk about abilities. e.g. 'Are you able to write formal letters?'

"Can" is used when I am "able" to do something or when I have the permission to do something. "I can't use my cell phone during work time" (I don't have the permission) , or "I can speak English fluently" (I'm able to speak) Could is the modal verb, used to talk about hypothetical situations or to be formal. "I wish I could go to London" or "I could go with you If I hadn't broken my leg" (hypothetical situation), or "Could you lend me your pen?" (formal, being polite)