定语从句的先行词就是主语吗？

Answer 1

不是的，定语从句的先行词就是定语从句所修饰的词，一般为名词，在句子中可以是主语、宾语、表语等名词性成分。

例如：
I saw a girl who is in a red dress. 我看到一个穿红裙的女孩。
这里 who 引导的定语从句修饰的是 宾语 girl,句子中I是主语。

He who works hard always ranks 1st in the class.认真学习的他总是班上的第一名。
这里who引导的定语从句修饰的是主语 he

There is the book which you are looking for.那就是你在找的那本书。
the book 跟在系动词is 后面，做表语，which修饰的正是表语the book.

以上的例子可以看出，定语从句的先行词不一定是主语。先行词是定语从句修饰的词，首先是个名词，所以在句子中做名词性成分的都可以，比如主语、宾语、表语。

希望能帮到你O(∩_∩)O

Answer 2

可以是主语，如 I like the teacher who speaks good English.

也可以是宾语，如· This the English teacher whom I like.

Answer 3

The
house
that
was
built
last
year
is
now
for
sale.
主语的定语从句：that
was
built
last
year；先行词：The
house
A
cat
that
runs
faster
than
a
rabbit
is
hard
to
find.
主语的定语从句：that
runs
faster
than
a
rabbit；先行词：A
cat
......