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)
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)
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.