Q: When I taught 8th-grade grammar back in the ‘70s, I used to tell my students that “may” meant permission, while “might” meant possibility. Is that no longer the case? I often hear the words used interchangeably now.
A: There are two issues here: “may” is an auxiliary verb meaning to be allowed or permitted, but it is also an auxiliary indicating likelihood or possibility (like “might”).
Let me explain this possibility business by quoting a section from my grammar and usage book Woe Is I:
“May is a source of our word maybe, and that’s a good clue to how it’s used. We attach it to another verb (happen, for example) to indicate the possibility of something’s happening. If we say something may happen, we mean it’s possible or even probable.
“Might is a slightly weaker form of may. Something that might happen is a longer shot than something that may happen. I may get a raise is more promising than I might get a raise.
“Although your dictionary will tell you that might can be the past tense of may, either one can be used in the present tense (She may break a leg; She might break a leg) or in the past (She may have broken a leg; She might have broken a leg). The form you choose depends on the degree of possibility and can radically change your meaning. A bulletproof vest may have saved him implies that he was saved. A bulletproof vest might have saved him implies that he wasn’t.
“There’s an exception to this rule of possibility. … If a sentence has other verbs in the past tense, use only might: She thought [past] she might have broken a leg … Eloise was [past] afraid they might lose everything … Frank said [past] he might leave early.”