在句中作主语的从句叫主语从句。主语从句是名词性从句的一种。从句通常由that, whether 或其他疑问代词引导。
o That he is still alive makes us very happy.
o That we are invited to a concert this evening is good news to us.
o That we need more practice is quite clear.
notes
o That在从句中不充当任何成分，也不表示意思，仅用来表示一个事实，that不可以省略。
that 从句作主语时，常用it作形式主语，
常见的句型：
o It+ be+ 形容词(obvious, true, natural, surprising, good, wonderful, funny, possible, likely, certain, probable, etc.)+ that 从句
o It is certain that the boy will do well in his exam.
=That the boy will do well in his exam is certain.
o It is probable that he told her everything.
=That he told her everything is probable
o It +be +名词词组(no wonder, a good thing, a pity, no surprise, a shame, a fact, an honor, good news) +that 从句
o It is a pity that they can’t go.
o It is no surprising that our team has won the game.
o It + be + 过去分词 (said, reported, expected, decided, announced, arranged, suggested, considered, thought )+ that 从句
o It is said that Mr. Green has arrived in
o It is decided that the meeting has been put off till next Friday.
o It + 特殊动词(seem, appear, happen, matter, turn out) +that 从句
o It seems to me that you object to the plan.
o It happened that I had been away when he called.
疑问词引导的主语从句
o 主语从句也可以由疑问词引导，如：who, why, when, where, how, what。这类主语从句语序通常是陈述句语序而不是疑问句语序。
Who she will invite to her birthday party is not my concern.
When the meeting will be held has not been decided yet.
Why he gave up singing is of great interest to reporters.
How the little boy got out of the cave remains a secret.
主语从句作主语往往被看作单数.
When and why Jack came to
How we can overcome the financial difficulties in such a short period of time is a big problem.
o Whether 引导的主语从句
o It makes a big difference whether you wear red or white to a wedding.
o Whether you come or not all depends on the weather.
o 主语从句中只能用whether，不能用if.
or not 通常可以直接跟在whether后面,或者在主语从句中的宾语或宾语补足语后面，或者省略.
由疑问词引导的主语从句,同样可以用 “it”作形式主语.
It is pretty well understood what controls the flow of carbon dioxide in and out of the atmosphere.
但是: 如果整句话谓语动词是be则不可以用it.
What he did was wrong.
不能改成It was wrong what he did.
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