本句的完整形式：1）He saw that three boys who were nervous were getting on the bus.
本句的簡潔形式：2）He saw three boys who were nervous getting on the bus.
基礎(chǔ)知識：一個主句或從句,一般只有一個謂語動詞（即有時態(tài)的動詞）.
句子分析：
1）He saw that three boys who were nervous were getting on the bus.
本句有三個謂語：saw（過去時）,were（過去時）,were getting on（過去進(jìn)行時）,所以本句有一個主句,兩個從句.
主句：He saw that three boys were getting on the bus.主謂賓,賓語是that引導(dǎo)的從句.
賓語從句：that three boys were getting on the bus ,做saw的賓語.
定語從句：who were nervous ,修飾boys.
2）He saw three boys who were nervous getting on the bus.
本句有兩個謂語：saw（過去時）,were（過去時）,（注意vgetting是非謂語動詞,沒有時態(tài),不能做謂語）,所以本句有一個主句,一個從句.
主句：He saw three boys getting on the bus.主謂賓補(bǔ)（getting on the bus補(bǔ)充說明賓語boys的狀態(tài).）
定語從句：who were nervous ,修飾boys.