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I have seen in so many place where they would have mentioned "You may not.." etc for the things people shouldn't do. For eg: in companies where USB is not allowed, they will mention like this "You may not use USB/CD etc", similarly "You may not download free softwares", "You may not tailgate the doors" etc.

These all things should be like this right "You should not use USB/CD etc", "You should not download free softwares" etc, this "May not" seems like saying "optional" it may indicate like "people may follow" etc

Can anyone guide me in this? I always get confuse in this.

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You may not is just a formal way of being more strict, than you should not. –  Manish Aug 12 at 16:16
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May is a [modal auxiliary verb], and all modals have at least two kinds of meaning -- one kind, called Epistemic /ɛpəs'tɛmɪk/, refers to logical possibility and necessity, e.g. This may be the place means it's possible that this is the place. The other kind of modal meaning, called the Deontic sense, refers to obligation and permission, and is social, not logical. This is a case of deontic may, which refers to permission -- She may attend the ball, provided she keeps her shoes on all night. –  John Lawler Aug 12 at 17:25
    
    
Thanks for the answer to all :) –  Pradeep Simha Aug 12 at 18:40
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"May not" is best avoided in writing, since it can be ambiguous between "must not" (as in, "You may not enter") and "might not" ("You may not see any difference"). In spoken English, the former would tend to have the emphasis on "not" and the latter on "may" but this is lost in writing. For example "John may not read this comment" could perfectly well mean either that he's forbidden to read it or that it's possible he won't read it. –  David Richerby Aug 13 at 0:11

2 Answers 2

up vote 10 down vote accepted

In this context, may means that you have permission to do something, so may not means you are not permitted to do something.

should is generally interpreted less strictly, as describing a desire or strong suggestion, rather than a requirement. So should not means that they would prefer or recommend you not do something, not that it's prohibited. For example, You should not go swimming during a thunderstorm.

Sometimes these can be conflated, though. You should not drive above the speed limit, because it's against the law and you might get a ticket.

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THIS USAGE OF may means that you have permission to do something. –  Edwin Ashworth Aug 12 at 18:32

It isn't the same meaning of "may" as in RFC 2119: in this usage, 'you may' doesn't mean "it is optional", instead it means "it is permitted".

"May" is used in polite language to distinguish between "can" (physically able) and "may" (have permission to).

For example, can I tell a lie? Yes, I have that ability.

May I tell a lie? No, I don't have permission to.

You should interpret "may not" as meaning, "must not".

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