The choice of which one to use is not based on which is more probable. "Might" is the past tense of "may." For this reason, it is used in subjunctive constructions, which are generally "iffier" than categorical constructions. This give the impression that "may" is more probably than "might."

I may come to the party.

I might have come if had been invited.

In common conversations, you sometimes hear people say, "I might come to the party," indicating a lower probability than "I may come to the party." According to the American Heritage Dictionary, it is not wrong to use it that way.

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Note added at 2003-08-19 17:59:56 (GMT)
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Commenting on Hamnet\'s comment below:

Some people use the two interchangeably, and some people have a habitual preference for one or the other, but such usage is hardly of any normative value.

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Note added at 2003-08-19 22:40:04 (GMT)
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Commenting on sarahl\'s comment below:

\"Might\" is the past tense of \"may.\" As such, it fits well in conditional constructions (the subjunctive mood), as in:

\"Had I known, I might have changed my mind.\"

But the subjunctive mood is not the only place where the past tense is appropriate. You can, for instance, use it in indirect (reported) speech, as in:

\"She indicated that she might still consider the offer.\"

To summarize:

\"Might\" is the past tense of \"may.\" It ought to be used wherever the past tense is called for, as in the subjunctive mood, indirect speech, or any constructions where the past tense would be appropriate.

Because of its common use in conditionals, it has given rise to the notion that it indicates less probability than \"may,\" and some people, possibly many people, use it with that understanding. The American Heritage Dictionary does not consider that an error.

Some people habitually use \"might\" regardless of grammar, and some people use \"might\" and \"may\" interchangeably, but such usages are not normative (at least not as yet).

might 2. The American Heritage® Dictionary of the English Language: Fourth Edition. 2000.
...forms: Past tense of may
1a. Used to indicate a condition or state contrary to fact: She might help if she knew the truth. b. Used to indicate a possibility or probability...

could, might. The Columbia Guide to Standard American English. 1993
...These auxiliaries can both suggest a possible outcome: She might [could] change her mind, and in many such uses they are interchangeable.

For events in the present or immediate future, use either may or might (I may [might] decide to go after all), but for past time, most Standard users still prefer only might, as in Yesterday I might have decided to stay home, not the increasingly encountered Yesterday I may have decided to stay home. Journalese is now peppered with may where until recently might has been solidly entrenched.


"may" means to have permission to do something. Example: The boss says I may occasionally leave early if I have no meetings and my work is completed.

"Might," in this context, means maybe one will do something. xample: I might leave early if my cold gets any worse.



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Note added at 9 mins (2003-08-19 17:31:38 GMT)
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please give us an example so that we can be more specific in your context - both can be used. For example, I may go to the party on Saturday night. Previous and following phrases could affect the choice.

Some more evidence to support the above. From "Practical English Usage" by Michael Swan (Oxford Univ. Press, ISBN 019431197X and 0194311988)

"'May' and 'might': the difference

'Might' is not used as a past form of 'may': both 'may' and 'might' are used to talk about the present or future. 'Might' is mostly used as a less definite or more hesitant form of 'may¿, suggesting a smaller chance -- it is used when people think something is possible but not very likely. Compare:

I may go to London tomorrow. (perhaps a 50% chance)
Joe might come with me. (perhaps a 30% chance)"

Other uses for these words are also discussed in the same section of the book. For example: 'may' for typical occurences. "The flowers may have five or six petals; colour may range from light pink to dark red." 'Might' is used this way only in the past. "In those days, a man might be hanged for stealing a sheep." 'Might' is used as the indirect speech equivalent of both 'may' and 'might'. Etc.

"Might" is the past tense of "may". They are auxilary verbs, as are "will" (past tense: "would"), "shall" (past tense: "should"), "can" (past tense: "could"), etc. That's right: these are all verbs, even though in use they don't appear to be anything more than modifiers. In fact, they are indeed modifiers: verbs that modify. Because this is almost exclusively their function, they are called auxillary verbs. "Might" is simply the past tense of "may":

GO:WENT :: MAY:MIGHT

Thus, I would say:

"I may go to the store," and not "I might go to the store," because the sentence is in the present tense.

HOWEVER:
"I might have gone to the store," is better than "I may have gone to the store," because in this sentence, the speaker is referring to past events. (Technically, this is the present perfect, but for all intensive purposes, you can think of this as the past).

Of course, in everyday, colloquial speech, a distinction is rarely if ever made. Both display probablility and all would be understood. However, if you want to stick to correct gramamr, then you should follow the guidelines that I have discussed.

Good luck!

-Dan

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Note added at 2003-08-20 15:06:03 (GMT)
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Correction:
I spelled \"grammar\" incorrectly on that last line up there. My bad.

-Dan