Why can't I use "You might not borrow my car."?

Question

Hello AllWhy can I use "might" in the first example, but not in the second?Example 1.He may not have heard you. Call him again.He might not have heard you. Call him again.---------Example 2.You may not borrow my car.*You might not borrow my car.

Paco2004 · Accepted Answer

Example 1.
1. He may not have heard you. Call him again.
2. He might not have heard you. Call him again.

"May" and "might" are used here as modals for conjecture. "May"(#1) is a bit stronger in the certainty of the conjecture.

Example 2.
1. You may not borrow my car.
2. *You might not borrow my car.

"May" and "might" are used here as modals for permission. "Might" is a form more hesitant than "may" than to ask someone for permission. So you can use "might" as the more polite form when you ask someone for permission. That is, "Might I borrow your car?" sounds more polite than "May I borrow your car?". But when you give permission to somebody, decisive forms sound more polite than hesitant forms. So you would say "Of course, you can" or "Of course, you may". Logically speaking, the hesitant form "No, you might not" should be used as a polite form for a negative answer, but actually such an expression is not idiomatic. In those cases, they would use other expressions like "Sorry, but I am afraid not" or "Sorry, but I'd rather you didn’t" as polite answers.

paco

CalifJim · Answer

"might not" is used to show the logical possibility that a certain situation is not so.
(If followed by "have", we interpret the situation as having applied in the past.)

He might not have heard you. = It is possible that he did not hear you.
You might not borrow my car. = It is possible that you will not borrow my car.

"may not", in contrast, has (at least) two meanings.

a - the same meaning as "might not".

He may not have heard you. = It is possible that he did not hear you.
You may not borrow my car. = It is possible that you will not borrow my car. [This interpretation is less likely than the one shown below.]

b - to show that permission is denied (This form cannot occur with "have", because you can't give permission to do something which has already occurred in the past.)

He may not have heard you. cannot mean He did not have permission to hear you.
You may not borrow my car. = You do not have (my) permission to borrow my car.

For much more on modal verbs, see:

CJ

Why can't I use "You might not borrow my car."?

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