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Hello,

I have to fill in the gap using "might/must".

1. Oh God! He is fallen from the ladder. I cannot be sure, but he -------------------- be seriously hurt.

As it is written "I cannot be sure" so "might" looks ok.

Could anubody help me?
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Since the choice is limited between "might" and "must", I'd choose "might".
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If you say He might be hurt you are saying It is possible that he is hurt.
If you say He must be hurt you are saying The only possible conclusion is that he is hurt, that is, There is no possibility that he is not hurt.

Given the statement I cannot be sure, only the first choice (might) can be used without creating a contradiction.

CJ
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Comments  
Oh God! He fell from the ladder. I cannot be sure, but he could be seriously hurt. In this context, COULD is more suitable. However, MUST also be possible. It depends on what you thought at the time may have happened. Could = likely and Must = highly probable.
 pieanne's reply was promoted to an answer.
Dear Hanuman,

It is my opinion that we may not say «must» in your example. It is a contradiction of «cannot be sure».

Kind regards, Emotion: smile

Goldmund
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 CalifJim's reply was promoted to an answer.