She may/might not be Anil's sister

Question

She may not be Anil's sister.

She might not be Anil's sister.

What's the difference?

Answer 1

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.

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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...

• For emphatic refusal may is used:

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

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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