Here are the rewritten sentences:

1) The hostage could not be found anywhere.
2) A fine modern building has been designed by him.
3) Ali has not only studied in the U.S. but also in Europe.
4) Laurence has not seen his sister since she left for Japan.
5) The food was so hot that I could not eat it.
6) My sister cannot cook as well as my mother does.
7) It took him two hours to repair the bicycle.
8) The car is going to be repaired by us next week.
9) Many buildings have been destroyed by the fire.
10) We couldn’t drive because of the fog.