简单地说，一句话中不只一个谓语的时候就肯定有从句。

1）主语从句：一个句子在复合句中充当一个主语，如：What we need is time.如果从句较长，谓语又较短，避免头重脚轻，可用it作形式主语，而将从句放在句末。如：It is said\eported\\estimated\\has been proved that......2）定语从句：定语从句修饰的先行词可以是名词或代词，也可是句子。定语从句通常位于先行词之后，由关系代词或关系副词引导。如：The girl whose parents died in an accident is living with her grandmother. 3）引导目的状语从句的连词有：so that,in order that,for fear that等，从句常使用may,might,can,could,would等情态动词。如： We got up early this morning so that we could catch the first bus to the railway station.