May, might, could, and must are used to show a degree of certainty in the present.
May not, must not, might not, can't be, and couldn't be are the corresponding negative forms.
Degrees of certainty in the present
To show degrees of certainty in the present we use modal + simple present.
Remember that the verb after a modal is in simple form so the structure is: modal + verb in simple form.
If you are very confident then use the simple present.
- The party is great.
If you are not completely certain then use must + simple present.
- The party must be great.
If you are around 50% certain use may + simple present.
- The party may be great.
If you are not very sure at all use might or could + simple present. These show that you doubt it.
- The party might be great.
- The party could be great.
Polite requests
Expressing necessity & obligation
Lack of necessity
Prohibition
Expectation
Giving advice
Making suggestions
Ability
Expressing preference
Degrees of certainty (present)
Degrees of certainty (past)
Degrees of certainty (future)
Progressive forms
Repeated past
Modal & phrasal verb combinations
To make any of them negative you can contract could with not, couldn't, or add not between the modal and the main verb. Couldn't, could not, must not, might not, may not.
Must can be contracted in casual speech but not in writing since it is only an informal contraction whereas couldn't is a normal, or formal contraction.
Obviously by making them negative the certainty is the opposite. Must is close to 100%. Must not is close to 0%. May is around 50%, May not expresses more doubt so 50% would be its limit.
- The party isn't great.
- The party must not be great.
- The party may not be great.
- The party might not be great.
- The party couldn't be great.
