Nobody can make it rain, so whether or not it rains is beyond anyone's control.
The modal "will" shows that the speakers is making a prediction with certainty.
She has something else planned for that evening,so she can't come to the party.
"If he could come" expresses less potential than "If he can come' .
It isn't certain but there's a good probability that they will finish at five.
We don't know for sure,but there is still a possibality that they can leave on Friday.
The modal "may" shows that the speaker thinks something is possibale.
One interesting rule is that no more than one modal can be used with any verb.
The modal "must" expresses logical or social necessity.
