定语从句中如果缺主语，宾语，表语要用关系代词，先行词为人用who,whom,that，为物用which, that.缺状语用关系副词when,where,why.

Have you finished reading the book which/that /\ I gave you yesterday?

I still remember the days when I worked in a small village.

第一句中定语从句I gave you yesterday 中give带双宾语，因此缺了一个宾语，所以用关系代词。第二句中，work为不及物动词，不需要带宾语，因此用关系副词，先行词是表时间的，所以用when。从句相当于：I worked in a small village in the days.