Question

12．刷牙时关水龙头是个好习惯．
It's a good habit to turn off the tap when you brush teeth．．

Answer 1

分析 It's a good habit to turn off the tap when you brush  teeth．

解答 答案：It's a good habit to turn off the tap when you brush  teeth．这里是表达客观事实用一般现在时，这里用含有when（当…时候）引导的时间状语从句的主从复合句来表达，主句用it作为形式主语，动词不定式做真正主语，故it's a good habit to．关水龙头turn off the tap，从句when you brush  teeth当你刷牙的时候，故答案是It's a good habit to turn off the tap when you brush  teeth．

点评 形式主语，顾名思义，即形式上是主语，但实际上不是真正的主语，真正的主语是后面的动词不定式、动名词或主语从句等．如果让后面的动词不定式、动名词或主语从句用作主语，那么在句式上容易造成"头重脚轻"，句式不平衡，故往往用it作形式主语，而把真正的主语放在后面．it在这里没有任何意义．
It's+名词+to do sth
It's everyone's duty to obey the law．
It is not a good habit to stay up too late．