Might have vs Must have

Express varying degrees of certainty about past events

 

What Happened?

MIGHT  HAVE
Might expresses an opinion: an uncertain guess, a speculation, based on very little information. Might have expresses an opinion about an earlier (past) situation.
A GUESS
The car driver might have tried to pass between two buses.
One of the bus drivers might have changed lanes without looking first.
The bus drivers might not have been able to see the smaller, lower car.
The road might have narrowed and so they all tried to move into the same lane.

MUST HAVE
Must expresses an opinion: an inference, a conclusion, based on known details.Must have expresses an opinion about an earlier (past) situation.
A CONCLUSION
The car driver must have panicked when he saw the buses moving in on both sides.
The bus passengers must have heard a crunch. (sound)
The car passengers must have escaped out the back.  (No other exits were available.)
The insurance company must have declared this car "totaled".

 

infer (V) – to form an opinion that something is probably true because of information that you have
inference (N) – an opinion that is formed on details or knowledge that you have
panic / panicked (V) – a sudden strong feeling of fear or nervousness that makes you unable to think clearly or behave sensibly
speculate (V) – to guess about the possible causes or effects of something, without knowing all the facts or details
totaled (Adj) – not worth fixing; totally gone

See Modal Review  "Degree of Certainty / Possibility"

Must

One word with two meanings

Conclusion vs. Necessity

MUST – CONCLUSION
Must have expresses a conclusion about what happened (past), and is based on putting details and clues together and making an inference (a sure guess).
Traffic officers must be accustomed to seeing occasional bizarre accidents like this one, because they see so many accidents each year
The passengers in the car must have been extremely frightened.

MUST – NECESSITY
Must expresses an opinion about what needs to be done  in the present or future, and is based on one's personal sense of what is right and what is wrong.
Drivers must not make unsafe lane changes. (present tense)
Each of the drivers had to file an accident report. (present tense)

 

Also see Was / Must have / Might have and Should have / Needed to

Sentence Types

Word Order

SENT TYPE	SUBJECT	MODAL	VERB BASE
STATEMENT
guess	She	might	be	upset.
inference	She	must	be	upset.
NEGATIVE
guess	She	might not	be	upset.
inference	She	must not	be	upset.
PAST
guess	She	might have	been	upset.
inference	She	must have	been	upset.
NEG. PAST
guess	She	might not have	been	upset.
inference	She	may not have	been	upset.
W / ADVERB
guess	She	might really have	been	upset.
inference	She	must truly have	been	upset.

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