由连接代词或连接副词引导的主语从句有哪些特点和使用要求？

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同学您好：由连接代词或连接副词引导的主语从句
a.放在句首
Whenhe’ll be back depends much on the weather.
How it was donewas a mystery.
b.用it做先行主语
Itwas not clear to me why he behaved like that.
It’suncertain whether the game will be held.
Itwas a mystery how the burglars got in.
Itwas a puzzle how life began.
Itdoesn’t matter much where we live.
Itmakes no difference whether we go by train or by boat.
Itis not yet settled whether I am going to America.
Itis not decided who will edit it.
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