几个基本原则：
1）介词后面的关系词不能省略。
2）that前不能有介词。
3）
某些在从句中充当时间，地点或原因状语的"介词+关系词"结构可以同关系副词when
和where
互换。
This
is
the
house
in
which
I
lived
two
years
ago.
This
is
the
house
where
I
lived
two
years
ago.
Do
you
remember
the
day
on
which
you
joined
our
club?
Do
you
remember
the
day
when
you
joined
our
club?
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