may expresses probability
if i'm free tomorrow, i may go.(here i'm not sure)
i'm free tomorrow, so i'll go to the theatre.(here i'm sure)
to express ability in future , use will be able to
when i was four, i could ride a bike
i can speak english now.
i will be able to drive a car when i leave school.
i think in the furure people will be able to live on the moon.