Can

We use can to express ability, possibility, permission and request.

  • The little boy can ride a horse faster than his father can. (Ability)
  • You can get into trouble for not doing your homework. (Possibility)
  • Can I walk on the grass? (Permission)
  • Can you not sing at all? I can’t stand it. (Request)
 
Could

We use could to express request, possibility, ability, permission and suggestion.

  • Could you feed my goldfish while I’m away for a week, please? (Polite request – a more polite form of can)
  • You had better not stand under the tree. You could be struck dead by lightning. (Possibility)
  • My grandfather could speak six languages including Swahili. (Ability)
  • Could I bring my puppy along? (Permission)
  • You could warn him to leave you alone, or you could keep a bodyguard. (Suggestion)
 
More on 'can' and 'could'

We do not use can with to-infinitive.

  • We can fly kites today.
  • Not: We can to fly kites today.

We can use can to express offer as is commonly done.

  • Can I help you to make the pancakes?

We can sometimes use (be) able to in place of can when it is necessary.

  • I can’t sleep. = I haven’t been able to sleep for two days. (Here, we use present perfect tense + able to. It is not possible to use can + present perfect tense.)

We do not add –s to the verb in the third person singular when we use the modal verb.

  • She can dance the tango.
  • Not: She can dances the tango.
 

We can use could +perfect tense for things which were possible to happen but didn’t, or an event which we are not sure about.

  • You could have avoided the fight by not using the word ‘stupid’.
  • He could have quarreled with his girlfriend.

We can use could when we do not really mean what we say.

  • I am so hungry; I could eat a whole turkey.
  • Not: I am so hungry; I can eat a whole turkey.