主語從句，即在復合句中充當主語成分的句子，例如"That he finished writing the composition in such a short time surprised us all"，他在很短的時間內(nèi)寫完作文讓我們都很驚訝，此復合句主要描述的是某事讓我們驚訝，某事就是該復合句的主語，因此"That he finished writing the composition in such a short time"便是該復合句的主語從句。

主語從句在復合句中做句子的主語。引導主語從句的詞有連詞that, whether; 連接代詞 who, what, which; 連接副詞 when, where, how, why等。接下來給大家羅列幾個經(jīng)典的。

如：

What is needed for the space trip is careful preparation.

Whether wild life can be well protected is of great importance.

Who will go makes no differences.

Why dinosaurs died out remains a puzzle.

Which kind of food is the best is still not certain.