主语从句，即在复合句中充当主语成分的句子，例如"That he finished writing the composition in such a short time surprised us all"，他在很短的时间内写完作文让我们都很惊讶，此复合句主要描述的是某事让我们惊讶，某事就是该复合句的主语，因此"That he finished writing the composition in such a short time"便是该复合句的主语从句。

主语从句在复合句中做句子的主语。引导主语从句的词有连词that, whether; 连接代词 who, what, which; 连接副词 when, where, how, why等。接下来给大家罗列几个经典的。

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.