定语从句中,在什么情况下省略关系代词

1、当关系代词为从句的动词宾语时和表语时；
2、there be的句子中；
3、先行词为the way时；
4、关系代词为从句的介词宾语而介词又在句末时。

1、在关系代词后面有主语，比如It is a book (which)I bought.
2、在关系代词后面又有联系动词加上进行时、被动或动词不定式 ，比如：It is a bike (which is) to be repaired. The boy (who is)running on the playground is my brother. There are some books (which are) sold in the bookshop.