(d)　先行词为序数词、数词、形容词最高级时，只用that。.　

(e)　先行词既有人，又有物时。

(f) 先行词指物，在主句中作表语时．

(g) 为了避免重复．

(h)先行词是the way时

（六）as,which引导的非限定性定语从句　

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。　

1）As we know, smoking is harmful to one's health.　

2）The sun heats the earth, which is very important to us.