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Modals or modal auxiliaries remain a little complicated for you…These strange verbs have no
‘real infinitives’, take no -s on the third person singular in the present, look and act different in
the interrogative and negative forms, do not exist in some tenses… Pffffff!!! 
Like all verbs (regular or irregular) they change with time and habits. Therefore, a few reviews
are always useful…
CAN is used to express what is possible to be done, but it has two values:
1) CAN is used to express a PERMISSION, or a CAPACITY ( both physical or intellectual).
– Guess what! I can go to your party! My parents have given their permission. (permission)
This action is possible because the young person has been given permission;
In this case, CAN may be replaced by TO BE ALLOWED TO (in the past, future and conditional.
– My parents will never allow me to go out on weekdays.
2) CAN may also express a CAPACITY. The person is capable of doing the action.
In this case, CAN may be replaced by TO BE ABLE TO.
– I can swim across this pool in seconds.
– When he repeated his question, I was able to understand what he wanted.
3) CAN’T, on the contrary, expresses an INTERDICTION or an INCAPACITY.
– ‘You can’t bring your cell phone in class. (INTERDICTION)
– He can’t walk at the moment and has a sprained ankle. (INCAPACITY) .
4) PROBABILITY:
CAN and COULD are used to express a probability:
– ‘Yes, it can sometimes rain here in summer, but it’s rare.’
– It could rain soon… Take your umbrella.
5) Asking QUESTIONS : COULD expresses a request more politely.
– Can you help him, please?
– Could you help him, please?
Don’t be afraid! You have the FORCE. Go for the test happily! 
End of the free exercise to learn English: Can/ could/ can’t and equivalents
A free English exercise to learn English.
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