定语从句是由关系代词或关系副词引导的从句，其作用是作定语修饰主句的某个名词性成分，相当于形容词，所以又称为形容词性从句，一般紧跟在它所修饰的先行词后面。
（1）When television was first introduced, the extent to which it could affect human society could not have been foreseen.
       人们发明电视的时候，并没能预见到它会对人类社会产生多大的影响。
（2）As is so often pointed out, knowledge is a two-edged weapon which can be used equally for good or evil.
       正如人们常常指出的，知识是一把双刃剑，既可以用于造福，也同样可以用来为害。
       定语从句按照关系代词的不同，会有不同的情况。