Modal verbs are used to show how likely things are to happen. This can be called expressing degrees of possibility.
It might rain today.
The parcel should arrive tomorrow.
Kevin's brother must come first in the race – he's such a fast runner!
Some adverbs can also be used to express degrees of possibility.
Perhaps it rained last night.
The parcel probably arrived yesterday.
Kevin's brother certainly has a good chance of winning the race – he's such a fast runner!
Sometimes both modal verbs and adverbs of possibility can be used in the same sentence for emphasis.
He probably ought to do it.
It definitely must be below freezing. I can see ice on the windows!
In this worksheet you can practise using modal verbs and adverbs to express degrees of possibility.