I understood that "might" represents a weaker possibility and "may" represents a stronger possibility. But I can't understand when I have to use "may not" and "might not". What's the difference between them?
Modal Verbs – Difference Between ‘Might Not’ and ‘May Not’
modal-verbs
Related Solutions
The two words have overlapping meanings, so there are contexts where either one could be used. However, there are certain constructs where one would be regarded as much better than the other.
I'm not sure if I can provide an exhaustive list of contexts where you should use might instead of could, or vice versa. Let's start with the example you provided, which is when either word is followed by not.
The phrase might not indicates a possibility that the contrary could happen. The word cannot suggests that the contrary won't happen. And the phrase could not suggests that the contrary wasn't able to happen.
So:
He might not show up.
means that he might show up, but we're starting to think he may not, while:
He could not show up.
suggests that, because of some obstacle, he was not able to make it.
Let's say we're supposed to meet Mike at 8 o'clock, and it's now 8:20. We have no idea where Mike is. I might say:
Let's not wait any longer. He might not turn up.
However, just before I finish my sentence, my cell phone rings. It's Mike! Mike tells me that his flight is delayed; the airplane is having mechanical problems, they haven't even left the runway yet. Mike is 500 miles away. Now, I say:
Let's not wait any longer. He cannot be here; he's stuck in Memphis.
Next, my friend and I leave, and we go to Ted's house. Ted sees that Mike his missing, and asks, "Wasn't Mike coming with you guys?" I might reply:
We waited for him, but he could not be here. His flight got delayed.
This is because, while might infers a possibility, could can infer a possibility, or an ability. Let's say you set a bar a meter off the ground, and ask me if I'm able to jump over it without touching it. I could say:
I might be able to do that. (it's possible)
But now you raise the bar two meters off the ground, and ask me if I can jump over it. I might say:
There is no way I could do that. (it's impossible)
However, there are times when the words could be used interchangeably, as you have sensed. For example, let's say we're leaving the house for a few days, and we're trying to decide whether or not to leave the furnace on. I might say:
Let's make sure the furnace gets turned on before we leave. We might get some cold weather.
or:
Let's make sure the furnace gets turned on before we leave. We could get some cold weather.
In that context, the two phrases mean essentially the same thing.
Now, an interesting example for the advanced reader. Consider these two statements:
The repairman might come today, but he could come tomorrow.
The repairman could come today, but he might come tomorrow.
Is there a difference? I'd say that both of them mean that the repairman is supposed to come either today or tomorrow, although there's a little uncertainty as to which day he will arrive. But does one of them suggest a higher probability one way or the other? I don't think so, at least not reliably.
He might have gone = There was a possibility that he had already gone. (past tense)
He might go (and I might go) refer to a possibility either in the past or the present. We thought he might go, but he didn't. I might go out later if it stops raining. He might go doesn't really fit with '...but luckily we reached his house and found him'.
Best Answer
Firstly lets get some perspective and correct your misconception. In some instances the two are directly inter changeable like with an introduction or a weak possibility. In others, like General Truths, they are not. In other situations, Permissions, Suggestions and the suggestion of a possibility might is the more polite form of may.
Hence in most situations if you are unsure of which to use I would err to the use of might over that of may
May maybe used as a possibility or a general truth.
May-Possibility
We use may to refer to a weak possibility in the present and future:
The economy may go up or down in the next year. (I think both are possible, the economy going up or the economy going down. I am not making either one a strong possibility.)
I think I may go to the doctor today and try to get some antibiotics. (I am not very sure yet if I will go to the doctor.)
May-General truths
We use may in formal writing, especially academic English, to describe things which the speaker thinks are generally true or possible. In this case, it is a more formal equivalent of can.
Might may be used as the past simple of the verb may, also as a possibility, permission, suggestion or as an introduction. It can also be used for what someone should do to be pleasant, correct, polite, etc, this context is usually associated with what someone would say when the are angry.
might modal verb (MAY)
past simple of the verb may, used especially when reporting what someone has said, thought, asked, etc.: I brought him some sandwiches because I thought he might be hungry. Very politely the little boy asked if he might have another piece of cake (= he said "May I have another piece of cake, please?").
might modal verb (POSSIBILITY)
Driving so fast, he might have had a nasty accident (= it could have happened but it did not). The rain might have stopped by now.
might modal verb (PERMISSION)
Might I ask a question? I wonder if I might have a quick look at your newspaper?
might modal verb (SUGGESTION)
You might try a little more basil in the sauce next time. I thought you might want to join me for dinner.
might modal verb (INTRODUCE)
used to introduce a statement that is very different from the statement you really want to make, in order to compare the two: The amount you save might be small, but it's still worth doing.
might modal verb (SHOULD)
You might at least try to look like you're enjoying yourself!
"I asked my boss over for dinner tonight." "Well, you might have asked me first!"
All references Cambridge English Dictionary Might may