26.　　 he felt sorry for was 　　 he wasn't in time to prevent the accident.

解析：前半句为what引导的主语从句，后半句为that引导的表语从句。

答案：What；that

25.　　 Thomas Edison became a great inventor is known to all.

解析：此为how引导的主语从句。

答案：How

24.The problem lies in 　　 we lack the time to have holidays.

解析：此为that引导的宾语从句，整个从句作介词in的宾语。

答案：that

23.　　 he wanted to see was an end to all the armies of the world.

解析：此为what引导的主语从句，同时what在从句中作see的宾语。

答案：What

22.Could you tell me for 　　 you have bought this fur coat？

解析：whom作介词宾语，介词for前置，原来位置应为：...you have bought this fur coat for(whom).

答案：whom

21.Lei Feng was always thinking of 　　 he could help others.

解析：how引导宾语从句，作介词of的宾语。

答案：how

20.The old woman was shocked by 　　had happened to her daughter.

解析：what引导的宾语从句作介词by的宾语。

答案：what

19.Mark wondered 　　 you thought of the new film.

解析：此为what引导的宾语从句，作thought的宾语。

答案：what

18.The chance 　　 he will attend the meeting is very little.

解析：此为that引导的同位语从句，在抽象名词chance之后，起补充说明的作用。

答案：that

17.　　there is life on another planet is almost impossible.

解析：此为that引导的主语从句。

答案：That