英语定语从句中什么情况下只用that，不用which？又在什么情况下只用which，不用that呢？

（1）当关系代词紧跟介词，作介词宾语时，只能用which或whom，不能用that。但当介词位于从句句末时，仍可用that或省略。如：
This
is
the
house
about
which
we
wrote
to
you.
Do
you
know
the
man
to
whom
your
brother
writes?
I
have
read
the
book
(which
/that)
you
told
me
about.
（2）当先行词是all,
everything,
something,
nothing等不定代词，或被first,
last,
only,
few,
much,
some,
no,
any以及形容词的最高级修饰时，只能用that，不能用which。如：
All
(that)
they
needed
were
qualified
workers.
This
is
the
most
difficult
book
(that)
I
have
ever
read.
（3）在非限制性定语从句中，只能用which,
who,
as，不能用that。非限制性定语从句既可修饰先行词也可修饰整个主句，往往起补充说明的作用，并有逗号与主句隔开。如：
He
was
ill
for
a
month,
which
set
him
back
a
lot
in
his
work.
I
want
to
buy
a
house,
which
has
a
garden.
有用请采纳，谢谢！

一.用that不用which
1）先行词是最高级或他前面有形容词最高级时
This
is
the
best
--that--
has
been
used
against
air
pollution
in
cities.
English
is
the
most
difficult
subject
--that--
you
will
learn
during
these
days.
2)先行词是序数词或它前面有一个序数词时
He
is
the
last
person
--that--
i
want
to
see.
3)主句中有疑问句
Which
is
the
pen
--that--
you
lost?
4)先行词既有人又有物时
The
bike
and
its
rider
--that--
had
run
over
an
old
man
were
taken
to
the
police
station.
5)先行词是
all,
much,
little
,something,everything,anything,nothing,the
one时
6）先行词前有
only,any,few,little,no
,all,one
of,the
same,
the
very等词装饰时
7）有2个定语从句，其中一个关系代词已用which，另一个应用that。
Tom
built
up
a
factory
which
produced
things
that
had
never
been
seen
before.
二.用which不用that：关系代词前有介词
This
is
the
school
in
--which--
you
will
study.
.原句本来就有that
Let
me
show
you
the
book
--that--
i
borrowed
from
the
library
--which--
was
newly
open.

关系副词when,
where,
why的含义相当于"介词+
which"结构，因此常常和"介词+
which"结构交替使用，这两者是完全相等的，并没有区别。例如：
there
are
occasions
when
(on
which)
one
must
yield.
任何人都有不得不屈服的时候。
beijing
is
the
place
where
(in
which)
i
was
born.
北京是我的出生地。
is
this
the
reason
why
(for
which)
he
refused
our
offer?
这就是他拒绝我们帮助他的理由吗？
前面用哪个介词，主要看把指代的词还原到从句中后搭配的介词。
比如上面的例子：
one
must
yield
(on
the
occasion)，所以用on
which
i
was
born
(in
the
place)
he
refused
our
offer
(for
the
reason).
你要是不确定，就不要用介词+which的形式，直接用when,
where,
why就好了。其实where和介词+which互换的比较多，日常英语中when基本不用which替代。