'may', in both, expresses a possibility, and since the truth value of a possibility is dependent on circumstances, both 1) and 2) are correct.
'maybe' is an adverb, whereas 'may be' is a modal plus a verb. They differ in function, carry the same meaning, but it's expressed in a different way. In  the meaning carried by 'may/be' is expressed as an afterthough. In , it is expressed as the main verb. Both express, I don't know all the circumstances, so I'm unsure as to whether that's really true or not:maybe A is true. What's the difference between them?
That may be true.
That is true, maybe.
I may not be able to play on Saturday. What could be th[at] sentence's context? Suppose the speaker is asked by his friend to play on Saturday. Is he saying this because he doesn't know for sure if he will be able to play, or because he knows for sure he could be unable to play?
The speaker uses 'may' to express there's a possibility (i.e., that s/he doesn't know if s/he will be able to play on Saturday. Circumstances will determine whether s/he can make the game or not. Maybe s/he has to work or might not have the car that weekend or maybe s/he doesn't want to play at all, so s/he uses 'may', expressing a possibility, as a polite way of getting out of playing that weekend.
Here's your chance to tell us what you know. What do you think?A vegetarian diet may not provide enough calories for a child's natural growth.
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