it引導的主語從句，形式主語就是it，真正的主語在句子的後面，通常是to do的不定式短語。
而強調句的結構 it is/was+被強調部分+that/who+句子其餘部分，結構詞it is/was和that/who都是沒有實際意思的，只是構成句子結構的詞而已。這個強調句只能強調名詞和代詞，也就是主語和賓語，不能強調其他句子成分。
你可以對比一下下面兩句：
主語從句：It is kind of you to help me with my housework.
強調句：It is you who help me with my housework.
主語從句中it做形式主語的常見句型
常見句型有：
1. It + be + 形容詞+主語從句，如：
It is uncertain whether he can come to Jenny’s birthday party or not.
2. It + be + 名詞片語 + 主語從句，這類名詞有：a fact, a pity, a shame, an honour，a question等，
如：It’s a pity that you missed the exciting football match.
It remains a question whether he will come or not.
3. It + be + 過去分詞+ 主語從句，這類過去分詞有：announced, believed, expected, hoped, decided, reported, said, shown等，
如：
It is reported that 16 people were killed in the earthquake.
It is not decided who will give the operation to the patient.
it做形式主語 必須用that引導從句嗎
不是必須..
it做形式主語，後面內容做句子真正主語，除了由主語從句充當之外，還可以由不定時，不定時短語，動名詞以及動名詞短語充當，
例如下面句子
It is very useful to learn English so well
It is the police"s duty to protect the peope"s life and properties
It is not so effective learning English without practising speaking English with others as often as possible
It is his duty taking good care of all his children.
it引導的主語從句，形式主語就是it，真正的主語在句子的後面，通常是to do的不定式短語。
而強調句的結構 it is/was+被強調部分+that/who+句子其餘部分，結構詞it is/was和that/who都是沒有實際意思的，只是構成句子結構的詞而已。這個強調句只能強調名詞和代詞，也就是主語和賓語，不能強調其他句子成分。
你可以對比一下下面兩句：
主語從句：It is kind of you to help me with my housework.
強調句：It is you who help me with my housework.
主語從句中it做形式主語的常見句型
常見句型有：
1. It + be + 形容詞+主語從句，如：
It is uncertain whether he can come to Jenny’s birthday party or not.
2. It + be + 名詞片語 + 主語從句，這類名詞有：a fact, a pity, a shame, an honour，a question等，
如：It’s a pity that you missed the exciting football match.
It remains a question whether he will come or not.
3. It + be + 過去分詞+ 主語從句，這類過去分詞有：announced, believed, expected, hoped, decided, reported, said, shown等，
如：
It is reported that 16 people were killed in the earthquake.
It is not decided who will give the operation to the patient.
it做形式主語 必須用that引導從句嗎
不是必須..
it做形式主語，後面內容做句子真正主語，除了由主語從句充當之外，還可以由不定時，不定時短語，動名詞以及動名詞短語充當，
例如下面句子
It is very useful to learn English so well
It is the police"s duty to protect the peope"s life and properties
It is not so effective learning English without practising speaking English with others as often as possible
It is his duty taking good care of all his children.