在句子中起名詞作用的從句叫名詞性從句(Noun Clauses)。名詞性從句的功能相當于名詞詞組，它在復合句中能擔任主語、賓語、表語、同位語、介詞賓語等，因此根據它在句中不同的語法功能，名詞從句又可分別稱為主語從句、賓語從句、表語從句和同位語從句。
1. 主語從句
(1)三種連接詞
①從屬連詞：that，whether等。
that引導主語從句只起引導作用，本身無實際意義，在主語從句中不充當任何成分，但不能省略。
That she left him cut him to the heart.
由whether及其他連詞引導的主語從句放在句首，句后都可。
Whether it will please them is not easy to say.
②連接代詞who，what，which，whatever，whichever，whoever。
What seems easy to some people seems difficult to others.
③連接副詞when，where，how，why等。
Why he did it remains a mystery.
(2)位置：主語從句可以前置，也可以后置。用it作形式主語，而把主語從句放在句末，常用下面幾種句型。
① It is still uncertain whether he is coming or not.
It + be + 表語 + 主語從句表語：(名詞、形容詞、過去分詞)
② It seemed (happened, doesn’t matter, has turned out) that…
It doesn’t matter whether she will come or not.
It + 不及物動詞 + 主語從句
③ It has been decided that the exhibition will not open on Sundays.
It is expected that the house price in Beijing will fall down.
It + 及物動詞(被動語態(tài))+ 主語從句
2.賓語從句
(1)由連接詞that引導的賓語從句
(2)主語+及物動詞+連接代詞/連接副詞引導的賓語從句
None of us knows where these new parts can be bought.
賓語從句的語序是陳述句語序，即：連接代詞/副詞+主語+謂語+其他成分。
(3)主語+及物動詞+whether/if引導的賓語從句，賓語從句要用陳述句語序，whether與if引導賓語從句時一般可以互換使用。
Please let us know whether (if) they will come to our party.
在discuss，wonder，not sure和介詞之后盡量用whether，if與whether含義容易混淆。
The board are discussing whether the fund should be allocated to that region.
(4)賓語從句中的否定轉移
若主句主語是第一人稱，謂語動詞為think，consider，suppose，believe，expect，guess，imagine等，其后的賓語從句若含有否定意義，一般要把否定詞轉移到主句謂語上，從句謂語用肯定式。
I don’t think this dress fits you well. I don’t believe you will finish the work today.
