Question

12．刷牙時(shí)關(guān)水龍頭是個(gè)好習(xí)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(dá)客觀事實(shí)用一般現(xiàn)在時(shí)，這里用含有when（當(dāng)…時(shí)候）引導(dǎo)的時(shí)間狀語從句的主從復(fù)合句來表達(dá)，主句用it作為形式主語，動(dòng)詞不定式做真正主語，故it's a good habit to．關(guān)水龍頭turn off the tap，從句when you brush  teeth當(dāng)你刷牙的時(shí)候，故答案是It's a good habit to turn off the tap when you brush  teeth．

點(diǎn)評(píng) 形式主語，顧名思義，即形式上是主語，但實(shí)際上不是真正的主語，真正的主語是后面的動(dòng)詞不定式、動(dòng)名詞或主語從句等．如果讓后面的動(dòng)詞不定式、動(dòng)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．