Question

12．刷牙時關水龍頭是個好習慣．
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．這里是表達客觀事實用一般現(xiàn)在時，這里用含有when（當…時候）引導的時間狀語從句的主從復合句來表達，主句用it作為形式主語，動詞不定式做真正主語，故it's a good habit to．關水龍頭turn off the tap，從句when you brush  teeth當你刷牙的時候，故答案是It's a good habit to turn off the tap when you brush  teeth．

點評 形式主語，顧名思義，即形式上是主語，但實際上不是真正的主語，真正的主語是后面的動詞不定式、動名詞或主語從句等．如果讓后面的動詞不定式、動名詞或主語從句用作主語，那么在句式上容易造成"頭重腳輕"，句式不平衡，故往往用it作形式主語，而把真正的主語放在后面．it在這里沒有任何意義．
It's+名詞+to do sth
It's everyone's duty to obey the law．
It is not a good habit to stay up too late．