定语从句在句中做的成分the room that there is a machine is a workshop in.这句话先行词在句中做什么成分?that was the day which/that ann left scholl on这句话先行词在句中作主语,怎么还原?为什么两句话都有介词在后面,但做的成分不同?

1。首先明确一个概念：定语从句所修饰的名词叫做先行词。比如你所给的句子中的room, day。
2. 你这里提出的问题实际上讲的是用关系代词代替关系副词引导定语从句。
我们知道一般情况下，引导定语从句的是关系代词（常用的有which, that, who等），修饰先行词。如：
I know the girl who is standing there.
This is the pen which/ that Mom bought me.
但是当先行词是表示地点或者时间或者原因的名词时，分别用关系副词where, when和why来引导定语从句。如：
This is the room where I was born.这就是我出生的房间。
I will never forget the day when I went to college.我永远忘不了我上大学的那一天。
3. 当关系副词引导定语从句时，可以用介词+关系代词来代替关系副词。比如：
This is the room which I was born in.
(可以介词提前变成：This is the room in which I was born.)
I will never forget the day which I went to college on. (I will never forget the day on which I went to college.)
这里注意两点：
a. 用什么介词取决于先行词或者谓语动词和这个介词的搭配，比如in the room; on the day。)
b. 如果用关系代词that的时候，介词就不能提前。不能说in that ...。
这样讲我想应该回答了你的问题。
补充一下：
你写的句子the room that there is a machine is a workshop in是不对的。应该改为
the room that there is a machine in is a workshop.或者
the room where there is a machine is a workshop.
或者the room in which there is a machine is a workshop.

the room （that there is a machine in ）is a workshop 作主语啊。
that was the day （which/that ann left scholl on）
先行词在句中作表语，

两句话都是介词加名词构成介宾短语作状语（第一个是地点状语,第二个是时间状语）
状语没法作主语,不能还原