that可以引导名词性从句、定语从句，用法比较复杂，其具体用法如下：
一、that引导名词性从句时（即主语、宾语、表语、同位语从句），在从句中无具体含义，只起连接作用，而且在从句中也不作任何句子成分。它所引导的从句的句子结构和意义是完整的，而且从句一般是陈述一个事实。例如：
①That
she
was
able
to
come
made
us
very
happy．她能来使我们很高兴。
②The
news
that
our
team
has
won
the
match
is
true．我们队赢了这场比赛的消息是真的。
③The
reason
he
didn't
come
was
that
he
was
ill．他没来的原因是他病了。
当that引导的主语从句或宾语从句太长时，常用it作形式主语或宾语而把that从句放在后面。如：
①It
is
well—known
that
the
earth
is
round．众所周知，地球是圆的。
②We
find
it
necessary
that
we
practise
spoken
English
every
day．我们发现每天练习英语口语很有必要。
that引导的主语从句，表语从句以及同位语中，that不能省略。that引导的宾语从句，一般可以省去that，但在下面情况中不能省略。
1.在suggest,
order等表示命令、请求、建议等动词后的宾语从句中，不能省。
2.由and或but所连接的两个引导的宾语从句时，第一个从句中的that可以省，第二个从句中的that不能省。
3.that引导的宾语从句，如果从句中还有其他状语从句，引导宾语从句的that不能省略。
Tell
him
that
if
he
is
at
home,
I’ll
call
to
see
him.
二、that引导定语从句要注意以下几点：
1．定语从句中缺主语，修饰人时用who
／that，修饰物时用which
／that引导。如：
Is
this
the
factory
which
／that
makes
TV
sets？
The
man
who
／that
visited
our
school
is
from
Australia．
2．从句中缺宾语，修饰人时用
who
／whom
／that或省略引导词；修饰物时用which
／that或省略引导词。如：
Gone
are
the
days
which
／that
we
spent
together
in
the
village．
He
is
the
man
who
／whom
／that
I
think
to
be
worthy
of
our
praise．
3、用that不用which的七种情况：
①先行词前有形容词最高级或序数词修饰时，用
that不用which引导。如：
This
is
the
best
place
that
I
have
ever
visited．
②先行词为little，much，all，none，few，（the）one，
something，everything，nothing，anything等不定代词时，用that引导。如：
There
is
nothing
in
the
world
that
can
frighten
him．
③先行词既有指人的名词又有指物的名词时，用
that引导。如：
He
told
us
about
the
people
and
the
cities
that
he
had
visited．
④先行词前有any，all，no，few，every，some，little，
much，（the）very，（the）only，（the）last等词修饰时，用
that引导。如：
This
is
the
very
coat
that
I
need．
⑤先行词为which时，为了避免重复，用that引导。如：
Which
is
the
book
that
you
bought
yesterday？
⑥先行词在从句中作表语时，常用that引导。如：
Shanghai
is
no
longer
the
city
that
it
used
to
be
in
the
1930's．
⑦当主句是There
be．．．句型时，用that引导。
There
are
two
tickets
for
the
film
that
are
for
you
two．
温馨提示：答案为网友推荐，仅供参考