what不能引导定语从句，只能引导
名词性从句（如
主语从句、宾语从句、表语从句）。如：
This
is
what
I
want
to
do.这就是我想做的事。
which
that
可以引导定语从句，并且都可以在定语从句中充当主语或宾语。
which指代的
先行词只能是物，that指代的先行词既可以是物，也可以是人。
在下列三种情况下只能用which，不能用that：
1、紧跟在介词后面。如：
This
is
the
very
pen,
with
which
I
have
written
many
articles.
2、which后面有
插入语。如：
This
is
the
gramma
book
which,
as
I
have
told
you,
will
improve
your
English.
3、引导
非限制性定语从句。如：
She
has
a
house,
which
was
built
last
year.
但在下列六种情况下只能用that：
1、先行词有形容词的最高级或
序数词修饰时，必用that。如：
It
is
the
first
shop
that
lies
at
the
foot
of
the
mountain.
2、在
不定代词，如：anything,
nothing,
the
one,
all,
much,
few,
any,
little等作先行词时，只用that。如：
All
that
haven't
been
done
must
be
done
at
once.
3、先行词既有人，又有物时，只用that。如：
We
finally
saw
the
school
and
teachers
that
we
haven't
seen
for
nearly
ten
years.
4、在there
be
句型中，只用that，不用which。如：
There
is
a
ruler
that
is
made
of
wood.
5、先行词有the
only,
the
very修饰时，只用that。如：
Lucy
is
the
only
girl
that
comes
from
America.
6、
关系代词在从句中作表语并带有类比含义时，
通常只用that，
不用which.
如：
He
is
no
longer
the
star
that
he
was.
他不再是过去的那位明星了。