如果定语从句的先行词就是指代前面的人或物【即在定语从句中作主语或宾语】，那么很多情况下可以转换成用现在分词词组或过去分词词组作后置定语，取代定语从句。如：
Do you know the man who stood at the gate just now.=Do you know the man standing at the gate just now?
The bridge which was built last year cost a lot of money.=The bridge built last year cost a lot of money.
如果定语从句本身是进行时或被动语态，基本上都可省略的。如：
The old man who is sitting under the tree is his grandfather.=The old man sitting under the tree is his grandfather.
The car which was bought last year has been stolen.=The car bought last year has been stolen .
扩展资料：
单词作定语时通常放在它所修饰的词之前，作前置定语。短语、从句作定语时则放在所修饰的词之后，作后置定语。
被定语从句修饰的名词、代词称为先行词。如“the girl”，“the book”
如:She is the girl who likes singing. 她就是那个喜欢唱歌的女孩。
这就是一个定语从句。
特殊情况：有六种情况只可用that:
（1）不定代词 anything, nothing, everything,all,much,few,any,little为先行词时；
Finally, the thief handed everything that he had stolen to the police. 最终，小偷偷的所有东西全部交给了警察。
（2）先行词为 the only, the very, the just时；
He is the very man that helped the girl out of the water. 他从水中救起了那个女孩。
（3）先行词为序数词、数词、形容词或最高级时；
The first English book that I read was "The Prince and the Pauper" by Mark Twain. 我读的第一本书是马克·吐温写的《王子与贫儿》。
（4）先行词既有人又有物时；
He talked about the teachers and school that he visited. 他在讲他以前访问过的学校和老师。
（5）当主句是以who或which开始的特殊疑问句时，为避免重复用that；
Who is the person that is standing at the gate? 那个站在门口的人是谁？
（6）关系代词作表语时；
He is not the man that he used to be. 他以前不是这样的人。
注：which在定语从句中指物，可作主语或及物动词或介词的宾语，作宾语时可省略。
that在定语从句中既可指人又可指物，在定语从句中作主语、宾语和表语，作宾语时可省略，指人时，相当于who或whom，指物时，相当于which，作介词宾语时，介词不可提到that前，当介词提前时，需要用which或whom来代替。
Do you know the man who stood at the gate just now.=Do you know the man standing at the gate just now?
The bridge which was built last year cost a lot of money.=The bridge built last year cost a lot of money.
如果定语从句本身是进行时或被动语态，基本上都可省略的。如：
The old man who is sitting under the tree is his grandfather.=The old man sitting under the tree is his grandfather.
The car which was bought last year has been stolen.=The car bought last year has been stolen .本回答被网友采纳