⑴ 做宾语的关系代词常可以省略；

⑵ 关系代词在定语从句中做主语时，从句谓语动词的数要和先行词一致；

⑶ 先行词为不定代词 all、something、nothing、anything、little等不定代词时或是序数词或是形容词最高级时，关系代词用that 来引导，强调句中也可用that,

   have you anything that you need?

   the first thing第一件事情that we should do is to clean打算 the room.

   it was the largest map that i ever曾经 saw.

   it was liberation 解放that brought about 引起带来a complete 彻底的change改变in his life.

⑷ 关系代词在定语从句中亦可用作介词的宾语，如: whom (指人)、which (指物)

   this is the book for which you asked.  寻找

   the person from whom the message came来自 did not say his name.

4、非限定性从句。

如果缺少了不会影响全句的主要思想，前面一般有逗号，翻译时常译成两个句子，that和why一般不用于非限定性定语从句，也不能省略任何关系副词。

sunday星期日 is a holiday假期, when people do not go to work.

i picked up捡起，拾起 the apples, some of which are badly bruised.碰伤