有一读者问：有这样两道选择题：
(1)They were the Boy Scouts_________to help elderly people cross the street.
A. whose was the task B. to whom it was the task
C. it was whose task D. whose task it was
(2)Those were the soldiers_________to save the town.
A. whose responsibility it was.
B. in whom there was a responsibility
C. whose was the responsibility
D. from whom the responsibility came
上述句（1）的答案是D, 句（2）的答案是A。请问如何分析这里的it? 将它删去行不行？
这里的it在语法上叫做引词或先行词。它本身无实义，只在句中起一种引导的作用。如引导的是主语，它就唤作形式主语；如引导的是宾语，它就唤作形式宾语。在上述两句中，它引导的都是用作主语的不定式短语（分别是to help elderly people cross the street和to save the town）。这样，it就是形式主语，不定式短语就是真实主语。整个从句的结构是：表语（分别为whose task 与whose responsibility）+形式主语（it)+连系动词(was)+真实主语（不定式短语）。如将从句变为独立句， 上述句（1）即可变为：
(3)They were Boy Scouts. It was their task to help elderly people cross the street.
同样，上述句（2）即可变为：
(4)Those were soldiers. It was their responsibility to save the town.
这样，你就可以更清楚地看到各个词语在句中所起的作用了。那么，可不可以将上述句（3）与句（4）中的it删去不用呢？ 答案是肯定的。理由很简单，因为上述句（3）与句（4）也可变为：
(5)They were Boy Scouts. Their task was to help elderly people.
(6)Those were soldiers. Their responsibility was to save the town.
不过请注意，whose task was to help elderly people cross the street 与whose responsibility was to save the town的结构则变为：主语（whose task 与whose responsibility)+连系动词(was)+表语（不定式短语）。
