除了关系副词（where/why/when),关系代词(which/who/whose/that/whom)在先行词作定语从句中作宾语的时候可省略． 如：
This book (that/which)i bought have been lent to my classmate.这里”书”作宾语,所以关系代词可省略．
但若前面有介词,则不可省略,如：
The girl with whom i went shopping is my sister.(也可说:The girl who i went shopping with is my sister.)