从句,也称为子句或附属从句,是复合句中不能独立成句、必须依附于主句才能表达完整意义的部分。例如在句子"I know that she likes music"中,"I know"是主句,而"that she likes music"是从句。从句必须依附于主句才能表达完整的意思。
从句按照在句中充当的成分可以分为三大类:第一类是名词性从句,它在句中可以作主语、宾语、表语或同位语,相当于名词的功能。第二类是定语从句,它在句中作定语,修饰名词或代词,相当于形容词的功能。第三类是状语从句,它在句中作状语,修饰动词、形容词、副词或整个句子,相当于副词的功能。
名词性从句在句中起名词作用,可以充当四种成分:第一,主语从句,如"What she said surprised me"中,"What she said"作主语;第二,宾语从句,如"I know that he is right"中,"that he is right"作宾语;第三,表语从句,如"The fact is that he failed"中,"that he failed"作表语;第四,同位语从句,如"The news that he won spread quickly"中,"that he won"作同位语。名词性从句常用that, if, whether, who, what等词引导。
定语从句用来修饰名词或代词,相当于形容词的作用。定语从句分为两种:限制性定语从句和非限制性定语从句。限制性定语从句对先行词进行限制,如果缺少它,句意就不完整,例如"The book that I bought yesterday is interesting",从句限定是"我昨天买的"那本书。非限制性定语从句只是对先行词进行补充说明,用逗号与主句隔开,例如"My father, who is a doctor, works in a hospital",从句只是补充说明"我父亲是医生"这一信息。定语从句通常由关系代词who, whom, whose, which, that或关系副词when, where, why引导。
状语从句用来修饰动词、形容词、副词或整个句子,相当于副词的作用。状语从句按照其表达的意义可分为九种类型:时间状语从句,如"I will call you when I arrive",表示动作发生的时间;地点状语从句,如"We can meet where the river meets the sea",表示动作发生的地点;原因状语从句,如"He was late because his car broke down",表示动作发生的原因;目的状语从句,如"I study hard so that I can pass the exam",表示动作的目的。此外还有结果状语从句、条件状语从句、让步状语从句、方式状语从句和比较状语从句。每种状语从句都有其特定的连接词。