谢谢曹老师出题，我试着作出判断，如有不妥和错误，请指正！

1 Behind them there stood the senators.

正确。其中的 there 也可以省略。（原句中的 senitors 估计是笔误，似应为 senators）

2 Behind them there stood the Vice President.

正确。其中的 there 也可以省略。

3 They are so nice boys.

错误：so 改为 such。“so + 形容词”后面要接单数可数名词（即要有不定冠词），如 He is so nice a boy.

4 All that heard her speak were deeply impressed by her.

错误：that 改为 who。all 指人时，其后的定语总是用 who 来引导。

5 Most of those who can, work at home.

正确。句子为 Most of those [who can work at home] work at home. 的省略形式，其中的 who can work at home 为定语从句，修饰 those。为简洁起见，省略成现有形式，且这样用的逗号不能省略，否则句子意思会有所有变化，且不完整。

6 The water seems colder today than was it yesterday.

错误。than 后面可以接倒装结构，但由于 than 后面的句子太短，且主语为代词，故不宜用倒装。