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She may not be Anil's sister.

She might not be Anil's sister.

What's the difference?

  • what do you think? – Maulik V Jun 9 '15 at 8:37
  • Just confirming... – Phoenix Jun 9 '15 at 9:07
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    that's what I'm asking... confirming what? – Maulik V Jun 9 '15 at 9:20
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    @MaulikV I think it's a straight question that deserves a straight answer. It's not one of those which requires research imo. – Araucaria - Not here any more. Jun 9 '15 at 11:39
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    I'm voting to close this question as off-topic because the OP accepted the first answer, and it is clearly wrong. That isn't the question's fault, but unless OP un-accepts the answer, we run the risk of having an incorrect answer memorialized permanently. – Adam Jun 9 '15 at 15:25
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Explanation for the examples given in the question:

May and might are used interchangeably, when expressing a possibility or uncertainty (a situation when we are not sure of something). This is the case in your example. We might imagine a conversation that goes like this:

A: Anil's sister came to pick up his things. Where did you put them?

B: Have you checked her ID? She may/might not be Anil's sister.

So, speaker B is not sure whether the visitor is Anil's sister.

In a different situation the same interchangeable usage would apply:

A: Who is that girl in a blue dress?

B: She may/might be Anil's sister.

Again, B is guessing, but isn't sure.

  • Some sources suggest that usage of may/might implies equal probability (in this example it is equally possible that the girl is or is not Anil's sister).
  • Other sources state that if something is less likely you should use might and if it's more likely you should use may (but this is a flexible rule).

  • In formal or academic language, may should be used to express possibility:

A Giant sequoia tree may reach the height of 85 meters.


Other cases where may and/or might are used:

  • Polite requests: both are used, but might is considered rare, or is used for very formal requests (this might be a BrE/AmE difference - see references 1, 2 and 3). But:

May I help you?

is far more common than 'Might I help you' and can be considered idiomatic.

  • For polite suggestions both are used:

Next time you might try... You may want to consider...

You may not borrow the car until you can be more careful with it.

Note that this is not likely to be the case with your examples; nobody can forbid someone to be someone else's sibling (such situation would be rather absurd).

  • For giving permission, may (and not might) is used:

You may enter.

Again, this is not the case when you say: 'She may be Anil's sister' - in that sentence nobody is giving her the permission to be Anil's sister; they are speculating who the girl is.

  • As the past tense of may in requests, might is used:

She asked whether she might get a few days off.

  • For the things that have possibly happened may have/might have is used, but for counterfactuals (things that could have, but actually didn't happen) might have is preferred (although may have is not unheard of):

I think that comment may/might have offended some people.

If they had lived in another time, their lives might have been different.

  • There are many idiomatic expressions that use may and/or might, such as:

may as well/might as well, where both are common; or be that as it may which you should probably use instead of 'be that as it might' since the former is considerably more common


References:

  1. Learn English- British Council - have a look at the exercise at this link, it's a good one.
  2. Merriam-Webster's Learner's Dictionary
  3. The Ultimate Phrasal Verb Book by Carl W. Hart
  4. Oxford Dictionaries Language matters
  5. Advanced Grammar in Use by Martin Hewings
  6. Cambridge Dictionaries Online

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