Another way to use the modal auxiliary can is in informally
asking and giving permission and in making requests.
Examples:
Can I leave early?
(Do I have your permission to leave early? [informal])
Yes, you can leave any time after 3:00.
(Yes, you have permission to leave any time after 3:00. [informal])
Can you tell me the time?
(request [informal]: Please tell me the time.)
Can you help me?
(request [informal]: Please help me.)
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The negative of can is cannot (one word), but cannot
is generally contracted to can't in speaking. Here, can't
(cannot) shows that someone does not have permission
or that someone is not able to do what is requested.
Examples:
I'm sorry, but you can't (cannot) leave early. The project
that you're working on needs to be finished.
(Someone does not have permission to leave early. [informal])
Unfortunately I can't (cannot) tell you the time because
I don't have a watch.
(Someone is not able to do what is requested--tell the time.)
I'd like to help you, but I can't right now. I have a meeting
in just a few minutes.
(Someone is not able to do what is requested--to help
another person.)
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Special Notes:
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Informal is not the same as impolite.
Informal speech is used in relaxed, friendly situations.
Formal speech is used to show respect. Impolite
speech is angry, rude, and probably insulting.
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Sometimes teachers use a kind of joke to show the
difference between can and may in asking permission:
student:
teacher:
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Can I get a drink of water?
Yes, you are able to, but no,
you do not have permission to.
(The teacher thinks that the
student is asking for permission
too informally and pretends to
understand the meaning of can
as ability, not as permission.)
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In requests, it's possible to use can with you,
but not with may:
not this:
but this:
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*May you help me?
Can you help me?
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not this:
but this:
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*May you tell me what time it is?
Can you tell me what time it is?
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not this:
but this:
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*May you tell me where
the Post Office is?
Can you tell me where
the Post Office is?
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