Can is used to express ability, possibility, permission and more.
- The boy can ride a horse faster than his father can. (Ability)
- You can get into trouble for smiling at every girl. (Possibility)
- You can finish the whole bowl of chicken soup if you like. (Permission)
- Can I help you to count the money? (Offer)
- Can you not sing at all? (Request)
- You can lie down on the couch even if you are not tired. (Suggestion)
Using (be) able to in place of can
Sometimes the phrase (be) able to can be used in place of can when it is necessary.
- I can finish it in two days. = I am able to finish it in two days.
- We can swim across the river. = We are able to swim across the river.
Could is a more polite form of can when used to make a request. The modal could can be used to express the following.
Can/could is used with infinitive without to.
- We can fly kites today.
Not: We can to fly kites today.
- You could be making a big mistake.
Not: You could to be making a big mistake.
The suffix –s is not added whether to the modal verb or the main verb in the third person singular.
- She can dance the tango.
Not: She cans dance the tango.
Not: She can dances the tango.
Could is often used together with the perfect tense (Could + perfect tense).
- You could have avoided the fight by not using the word ‘stupid’.
- He could have come along if he wanted to.
- The home team could have won the match by not committing so many fouls.
- I couldn’t have known if no one told me about it.
- We couldn’t have got it right without the map.
Could can be used to not really mean what is said.
- I am so hungry, I could eat a whole turkey.
Not: I am so hungry; I can eat a whole turkey.