Modals may can should
Modals change the definition of a sentence. Modals are used with regular verb structures but change the sense of a sentence adding obligation, permission, possibility.

May: permission.
Can: possibility.
Should: obligation.

Structure
Regular sentence: Subject + modal + verb. I should see Jack.
Negative: Subject + modal + not + verb i should not see Jack.
Question: Modal + subject + verb? Should i see Jack?
Negative:
should not / shouldn’t
cannot / can’t
could not./ couldn’t

Examples

You may see John.
Should we try?
We can not go.