Question

定语从句中，先行词为the one 时，关系代词是不是只能用that

Answer 1

如果先行词the
one在句子（主句或从句都行）中作表语，则该问题为肯定答案。
定语从句中只用that，不用which的情况：
1.
先行词是all,much,little,few,something,
anything,everything,none,onthing
等不定代词。
2.
先行词是to
be
后面的表语，或者关系代词本身在从句中作表语。
例如
The
school
is
quite
different
from
the
one
that
it
used
to
be.
3.
主句以here,
there开头。
例如
Here
is
the
DVD
player
that
you
have
been
looking
for.
4.
先行词有the
only,the
same,
the
last
等副词修饰。
5.
先行词是序数词，形容词最高级或被序数词或形容词最高级修饰。
6.
主句是以who,
which
开头的特殊疑问句
例如
Who
is
the
person
that
is
standing
at
the
gate?
7.
先行词是两个或两个以上，分别表示人和物
例如
I
am
going
to
talk
about
the
countries
and
people
that
I
have
visited.

Answer 2

在限制性定语从句中，先行词被so,as,such,the
same修饰时，关系代词只能用as,
as在从句中充当主语、宾语或表语。
you
can
wait
for
me
at
the
same
place
as
we
did.
he
is
no
longer
the
same
boy
as
he
used
to
be.
he
is
such
a
clever
boy
as
every
teacher
likes.
i
have
never
seen
so
clever
a
boy
as
he
(is).