五、as, which 非限定性定语从句 

     由as, which 引导的非限定性定语从句，as和which可代整个主句，相当于and this或and that。As一般放在句首，which在句中。例如：
     As we know, smoking is harmful to one’ s health. 如我们所知，吸烟有害健康。
     The sun heats the earth, which is very important to us. 太阳使地球暖起来，这对我们人类很重要。
     as可引导非限制性从句，常带有“正如”的意思。
     As we know, smoking is harmful to one’ s health. 如我们所知，吸烟有害健康。(as作know的宾语)
     As is known, smoking is harmful to one’ s health. (as作从句的主语，谓语动词know要用被动式。)

六、关系代词与关系副词的选择

     用关系代词还是关系副词引导定语从句主要看关系词在定语从句中的作用（即所担当的句子成分）。试比较：
A.  I know a place where we can have a picnic. 我知道一个我们可以野炊的地方。
     I know a place which / that is famous for its beautiful natural scenery. 我知道一个以自然景色优美而闻名的地方。
B.  I'll never forget the days when I worked together with you. 我永远忘不了我们一起工作的日子。
     I will never forget the days that / which we spent together. 我永远忘不了我们一起度过的日子。
C.  This is the reason why he was dismissed. 这就是他被解雇的原因。
     This is the reason that / which he explained to me for his not attending the meeting. 这就是他向我解释的他没有参加会议的原因。