What may happen? And what must happen? To what extent are they opposites. “May” is permissive whereas “Must” is compulsory.
If we fly a kite, for the most part we can permit the kite to “take off if appropriate wind, movement, etc. overcomes the gravitational force caused by the weight of the kite. But We can pull the kite back down and it "must" come down (provided the chain of causation from pulling in the line to pulling down the kite is not broken). Thus the kite may go up i.e. permissive but it must come down.
A similar may--must relationship can exist when riding a bicycle in that holding the pedals stationary on a multi-speed permits the bike to move forward, faster or slower creating a “may” situation i.e. it may move ahead.
But by applying pressure to the pedals, assuming that we have sufficient torque to overcome friction, gravitational and other forces, the bike must move forward (short of some mechanical defect, accident, etc.). This creates a must situation.
Now while all of this seems a bit obvious, how often, for instance, do we watch dramas on TV such as detective shows where investigators receive “may” clues i.e. clues which may suggest factors, traits, etc. which cause investigators to believe that they can "type” or limit the perpetrator. For instance, they figure the car must be green since that was reported. Another recalls seeing letters “BAT” on the license plate. From the stab wounds. They figure the perpetrator was left handed and between 6' and 6'4" tall and that he has a fascination with lingerie. They convert the "mays” to “must things and often limited their search parameters to solving the crime with these hints, never assuming the slightest possibility that they might be wrong about anything. Converting may possibilities to “must certainties” without appreciation of the mental “slight of mind” can be a very dangerous occupational hazard. Fortunately the show is in a controlled environment i.e. on TV where even rare and artificial outcomes can entertain the TV audience.