9.I could have arrived here on time，but my car broke down on the way.

10.He must have finished his homework，for he is playing football now.

Ⅱ.单句改错

1.Jackson mustn't be a policeman，for he's much too short.mustn't→can't

2.He should have completed his work；otherwise，he wouldn't be enjoying himself by the seaside.should→must

3.The weather turned out to be fine yesterday.I needn't have take the trouble to carry my umbrella with me.take→taken

4.You couldn't have woken me up；I don't have to go to work today.couldn't__→needn't

5.We must have studied last night，but we went to the concert instead.must→should