MUST HAVE is a stronger way of saying MIGHT HAVE. You are more certain of what you are saying.
‘The dog’s ill. He might have eaten some rat poison.’ (Possibility)
‘The dog’s ill. He must have eaten some rat poison.’ (Strong Probability)
In the same way, CAN’T HAVE is a stronger version of MIGHT NOT HAVE:
‘Man might not have landed on the moon. ’ (Some doubt)
‘Man can’t have landed on the moon. The technology at that time made it impossible.’ (Strong Improbability)
Might’ used when a native Spanish speaker would say:
(‘Where’s Juan?’)
‘I don’t know. Perhaps he went to the dentist.’
This is grammatically correct, but a native English speaker would say:
‘I don’t know. He might have gone to the dentist.’
PERHAPS + PAST = MIGHT HAVE + Past Participle
‘May have..’ is identical to ‘Might have…’, but much more formal. It is not so commonly used, and it does not have the sense of permission (‘May I come in?’) that exists in the present.
‘Can’t have…’ and ‘Must have’ are suppositions. They do not have the sense of ability or obligation that they do in the present:
‘It must have been Simon’s brother – we don’t know him because he lives in Scotland.’
Could, Might, Can’t and Must
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