Modal Verbs
Usage of Can and Could
1. Indicates ability or objective possibility, and can also express requests and permission.
Can you finish this work tonight?
Man cannot live without air.
— Can I go now?
— Yes, you can.
Note: Could can also express requests, with a more polite tone, mainly used in questions, not in affirmative sentences; responses should use can (i.e., could cannot be used in the simple affirmative response in the present tense). For example:
Could I come to see you tomorrow?
Yes, you can. (The negative response can be No, I’m afraid not.)
2. Indicates astonishment, doubt, or disbelief.
Mainly used in negative sentences, questions, or exclamations.
Can this be true?
How can you be so careless!
This cannot be done by him.
3. The interrogative or negative form of “can(could) + have + past participle”
Indicates doubt or uncertainty about actions that occurred in the past.
He cannot have been to that town.
Can he have got the book?
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