Must have been, may have been, might have been, and could have been are used to show a degree of certainty in the past.
Couldn't have been, can't have (been), must not have (been), may not have (been), and might not have (been) are negative.
Degrees of certainty in the past
To show degrees of certainty in the past we use modal + present perfect.
Since the verb that follows a modal is in simple form the structure is: modal + have + verb in past participle.
If you are very confident then use the simple past.
- The party was great.
If you are not completely certain then use must + present perfect.
- The party must have been great.
If you are around 50% certain use may + present perfect.
- The party may have been great.
If you are not very sure at all use might or could + present perfect . These show that you doubt it.
- The party might have been great.
- The party could have been 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 have. 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 wasn't great.
- The party must not have been great.
- The party may not have been great.
- The party might not have been great.
- The party couldn't have been great.
