将一句话分成两个句子，主干句和跟从句eg:我特别讨厌我遇见他的那个城市。<1> I really hate the city. 我特别讨厌那个城市。<2> I met him in the city.我在那个城市遇见他。
将两句合成一句话I really hate the city I met him in the city.
找出同类项，将后者同类the city找出，待用关系词代替I really hate the city I met him in the city.
合并同类项，选择关系词相同的名词/代词为先行词，将后者同类词用关系词取代，属性为人或者拟人用Who属性为物用which。I hate the city I met him in which.关系词一般跟在先导词后面，整理为I hate the city in which I met him.
where本身作为地点副词可以描述地点，为了精简，则in which可以替换为where。简化为：I hate the city where I met him
我都忘记你过来的日子了。<1>分我忘记日子了。你在那个日子来的。<2>合I have forgot the day you came on the day.I have forgot the day on which you came.I have forgot the day when you came则关于时间的定语从句，如有“介词+which”可以用“when”代替，时间不等于when，必须“介词+which”。
我有一条眼睛是蓝色的小狗。<1>分 I have a dog.A dog’s eyes are blue.<2>合I have a dog a dog’s eyes are blueI have a dog who’s eyes are bluewho的所有格为whose则句子变成I have a dog whose eyes are blue
我不理解他离开我的原因。<1>分我不理解原因。他因为这原因离开我。<2>合I don’t understand the reason he left me for the reason.I don’t understand the reason he left me for which.因Why可以描述原因，则句子变为I don’t understand the reason he left me why.
我认识那个你喜欢的男孩。<1>分：I know the boy.You like the boy.<2>合I know the boy you like the boy. I know the boy who you like.the boy处于句子宾语位置，则用who的宾格形式whom则句子变为I know the boy whom you like.
地点不等于where，必须“介词+which”
时间不等于when，必须“介词+which”。