We should carry some extra bags. (possibilty, future tense, obligation, advice)
Should - advice
We _____ have a surprise test today.
may/might
We _____ save water. (convey advice)
should
He _____ drive fast. (convey ability)
can
I might fail this test. (ability, future tense, possibility, obligation)
Might - possibility
May I ask a question? (permission, possibility, past habit, ability)
May - permission
We ____ obey the rules as good citizens. (Convey Obligation)
must
You should eat healthy food. (possibilty, advice, future tense, obligation)
Should - Advice
He can swim very well. (possibility, necessity, future tense, ability)
can - Ability
She used to play outside. (possibilty, past habit, ability, obligation)
Used to - past habit
I shall go to the museum tomorrow. (future tense, possibility, ability, obligation)
Shall - future tense