可以一句话讲清楚的，通常没必要长篇大论，因为再好的 listener 或 reader 也是有度的。英语中有些句子非常的冗长且费解，令人望而生畏，所以语法功底好的人常常是用简明且又不会被曲解的表达方式，特别是定语从句。不善用定语从句的省略规则的人，往往写出来的文章不流畅且有拖泥带水的感觉。
定语从句在句中通常修饰名词或名词短语，例如以下例句中的 table 就是被修饰的名词(先行词）：
The table that he bought was for his kitchen.
他买的那张桌子是厨房用的。
其中的 that 可以省略，变为以下句子：
The table he bought was for his kitchen.
但并不是所有的关系代词都可以省略，定语从句的省略规则通常是通过以下几个的短语或成分转换实现的：
一、现在分词短语（主动语态）
如果关系代词在从句中作主语，那么从句可以改为现在分词短语（-ing）或过去分词短语（-ed），且任何一个主动语态都可以改为现在分词短语，包括一般现在时，一般过去时，现在进行时，现在完成时，改写方法是：去掉从句的关系代词和个别的助动词（如果有的话），然后用动词的 -ing 形式；如果从句是否定句，那么在动词的 -ing 形式前加 not，例如：
1、肯定句
a、每天锻炼的人减重最多。
The people that exercised every day lost the most weight.
The people exercising every day lost the most weight.
解析：直接去掉关系代词 that，然后把 exercise 改成 exercising。
b、有许多员工工作非常努力。
There were many employees who were working very hard.
There were many employees working very hard.
解析：直接去掉关系代词 who 及助动词 were，然后把 work 改成 working）。
c. 最终投票的议员们将离开议会。
The MPs, who have finally voted, are leaving parliament.
The MPs, having finally voted, are leaving parliament.
解析：直接去掉关系代词 who，然后把 have 改成 having；注意这里的助动词 have 不能去掉，因为是现在完成时。
注意：在非限制性定语从句中，现在分词短语（省略的定语从句）还可以放在句首并以逗号隔开，在句中作状语，例如：
Having finally voted, the MPs are leaving parliament.
2、否定句
a、不每天锻炼的人体重减轻得最少
The people that did not exercise every day lost the least weight.
The people not exercising every day lost the most weight.
解析：直接去掉关系代词 that及助动词 did，然后把 exercise 改成 exercising，并在它之前加 not。
二、过去分词短语（被动语态）
1、在一般时态中，去掉关系代词和 be 动词，留下过去分词；如果从句是否定句，那么在过去分词前加 not，例如：
a、尼日利亚的一位医生发现了这种新的疾病，目前还没有命名
The new illness, which was discovered by a doctor in Nigeria, has not yet been named.
The new illness, discovered by a doctor in Nigeria, has not yet been named.
解析：直接去掉关系代词 which 和 was.
b、从泰国进口的丝绸很值钱
The silk, which is imported from Thailand, is worth a lot.
The silk, imported from Thailand, is worth a lot.
解析：直接去掉关系代词 which 和 is.
2、在进行时态中，去掉关系代词和 be 动词，留下 being + 过去分词，例如：
这座由金斯敦开发商建造的大楼应该在5月份完工
The building, which is being built by Kingston Developers, should be completed by May.
The building, being built by Kingston Developers , should be completed by May.
解析：直接去掉关系代词 which 和 is.
3、在完成时态中，把 have 或 had 替换成 having，最终留下 having been + 过去分词/其他，例如：
由TMC建筑师有限公司设计的新市政厅预计将获得多个奖项
The new Town Hall, which has been designed by TMC Architects Ltd, is expected to receive several awards.
The new Town Hall, having been designed by TMC Architects Ltd, is expected to receive several awards.
解析：直接去掉关系代词 which，然后把 has 改成 having.
注意：同样省略后形成的过去分词短语也可以放在句首并以逗号隔开，在句中作状语，例如：
Having been designed by TMC Architects Ltd, the new Town Hall, is expected to receive several awards.
注意：在完成时中，若要强调从句的动作先于主句的动作时用过去分词，否则一般用现在分词，例如：
The students that have studied the hardest have the best results.
a、学习最努力的学生成绩最好
The students having studied the hardest have the best results.
解析：强调 hard studying 先于 best results.
b. 学习最努力的学生成绩最好
The students studying the hardest have the best results.
解析：没有强调 hard studying 先于 best results.
三、过去分词
当从句中只有 be 动词或 have 和过去分词时，把过去分词移动到所修饰的名词（先行词）之前，例如：
1、那辆坏掉的车停在马路边缘
The car, which had broken down, was by the edge of the road.
The broken down car was by the edge of the road.
解析：直接把 broken down 移到被修饰词 car 前面。
2、这个很脏的玻璃杯需要清洗
The glass, which is stained, needs cleaning.
The stained glass needs cleaning.
解析：直接把 stained 移到被修饰词 glass 前面。
四、介词短语
若关系代词后面接介词短语，那么直接去掉关系代词和 be 动词，例如：
1、桌上的那本书是我的
The book that is on the table is mine.
The book on the table is mine.
2、街对面的房子是用来出售的
The house which is across the street is for sale.
The house across the street is for sale.
3、穿夹克的那个男人是我的堂弟
The man who is in the jacket is my cousin.
The man in the jacket is my cousin.
五、形容词或形容词短语
1、若定语从句中只有一个形容词和 be 动词，那么直接去掉关系代词和 be 动词，然后把形容词移到被修饰词前面，例如：
a、年龄最大的男孩赢得了比赛
The boy who is oldest won the race.
The oldest boy won the race.
b、你能搬动最轻的箱子
You can carry the box that is lightest.
You can carry the lightest box.
c、我们去最热的国家吧
Let's go to the country which is the hottest.
Let's go to the hottest country.
2、若定语从句中有非 be 动词的动词存在（通常是感官动词 sense，smell，look，feel 或 sound 等等），把它改写成形容词短语，格式是：adj. + v-ing，例如：
a、发恶臭的鸡蛋不能吃
The egg, which smelled foul, couldn't be eaten.
The foul smelling egg couldn't be eaten.
b、我播放好听的音乐
I played the song that sounded the best.
I played the best sounding song.
c、他总是穿看起来昂贵的衣服
He always wears clothes that look expensive.
He always wears expensive looking clothes.
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