if引导的条件状语从句的用法

学习" if " 引导的条件状语从句的用法,现在总结一下:
if 条件句不一般,几个要点记心间;
条件句,放在前,逗号要放句中间。
条件句表可能，主句多用将来时；
条件句表事实，主句常用现在时。

在 if 引导的条件状语从句中，如果从句谈论的是一个有可能发生的事实及其产生的相关的结果，主句用一般将来时态，从句用一般现在时态。如：

We can walk there if we can't find a bus .
If it rains tomorrow ,we will not go to the zoo.
What will you do if you find a panda in danger.

如果 if 条件句谈论的是重复发生和预示要发生的情景和事件，则主从句大多用一般现在时态。如：
If bears are in danger ,they attack people.

注意 ：
在 if 引导的条件状语从句中，if 和条件句位置灵活，可直接放在主句后面，若 if 条件句放句首，
从句后面要加逗号，和主句隔开。
还要注意前后时态一致原则本回答被提问者采纳

1、主将从现 （主句为一般将来时，从句为一般现在时）；
eg: If it rains(从句) , I will stay at home（主句）.
=I will stay at home if it rains.
2、主句含有情态动词 （must、may、can etc.）从句为一般现在时；
eg: If you drive too fast , you may have an accident.
=You may have an accident if you drive too fast
3、主句为祈使句，从句要用一般现在时态；
eg: Don't play computer games if you don't finish your homework.

备注：
1、If 于句首时中间用逗号隔开，主句后不用逗号。
2、if 可用unless(除非...) 替换。 unless= if... not...
3、注意虚拟语气 主句和从句都为一般过去式
eg: If I was a bird , I could fly.（事实我不可能是鸟）

一.宾语从句的定义
置于动词、介词等词性后面起宾语作用的从句叫宾语从句。宾语从句的语序必须是陈述语序。谓语动词、介词、动词不定式，v.-ing形式后面都能带宾语从句。有些形容词（afraid,sure,glad等）之后也可以带宾语从句。
二.宾语从句中引导词的用法在复合句中作主句的宾语，引导词有：
连词：that
(that
常可省略),whether,
if
代词：who,
whose,
what
,which
副词：when
,where,
how,
why
等。
（一）that引导的宾语从句（在非正式场合that可以省略）
可跟that从句做宾语的动词有：
say,
think,
insist,
wish,
hope,
demand,
imagine,
wonder,
know,
suppose,
see,
believe,
agree,
admit,
deny,
expect,
explain,
order,
command,
feel,
dream,
suggest,
hear,
mean,
notice,
prefer,
request,
require,
propose,
declare,
report等。
例句：The
boy
believes
that
he
will
travel
through
space
to
other
planets.
注意事项：当主句谓语动词是
think,
believe,
suppose,
expect
等词，而宾语从句的意思是否定时，常把否定转移至主句表示。
例句：I
don’t
think
it
is
right
for
him
to
treat
you
like
that.
在以下情况中that不能省略
当句中的动词后接多于两个由that引导的宾语从句时，第一个that可省，但后面的that不可省。例句：He
said
(that)
you
were
too
young
to
understand
the
matter
and
that
he
was
asked
not
to
tell
you.
当主句的谓语动词与that宾语从句之间有插入语时，that一般不可省。例句：Just
then
I
noticed,
for
the
first
time,
that
our
master
was
wearing
his
fine
green
coat
and
his
black
silk
cap.
当that从句是双宾语中的直接宾语时，that不可省。例句：I
can’t
tell
him
that
his
mother
died.
注意事项：许多带复合宾语的句子，that引导的宾语从句经常移到句子后部，而用it作形式宾语。
例句：I
find
it
necessary
that
we
should
do
the
homework
on
time.
（二）由whether,if
引导的宾语从句
由whether(if)引导的宾语从句，实际上是一般疑问句演变而来的。意思是“是否”。宾语从句要用陈述句语序。一般说来，在宾语从句中whether与if可以互换使用，但在特殊情况下if与whether是不能互换的。
例句：I
wonder
whether(if)
they
will
come
to
our
party.
只能用whether，不能用if引导的宾语从句
在带to的不定式前例句：We
decided
whether
to
walk
there.
在介词的后面例句：I’m
thinking
of
whether
we
should
go
to
see
the
film.
在动词后面的宾语从句时例句：We
discussed
whether
we
had
a
sports
meeting
next
week
直接与or
not连用时例句：I
can’t
say
whether
or
not
thet
can
come
on
time.
只能用if不能用whether引导的宾语从句
if引导条件状语从句，意为“如果”例句：The
students
will
go
on
a
picnic
if
it
is
sunny.
if引导否定概念的宾语从句时例句：He
asked
if
I
didn’t
come
to
school
yesterday.
引导状语从句even
if（即使）和as
if（好象）时例句：He
talks
as
if
he
has
known
all
about
it.
（三）连接代词和连接副词引导的宾语从句
这样的宾语从句实际上是由特殊疑问句变化而来的，宾语从句要用陈述句语序。用于这种结构的动词常常是：see,
say,
tell,
ask,
answer,
know,
decide,
show,
find
out,
imagine,
suggest,
doubt,
wonder,
discover,
understand,
inform,
advise等。
英语中的连接代词有：who,whom,whose,which,what,在句中担任主语、宾语、定语或者表语。
例句：Can
you
tell
me
whom
you
are
waiting
for?
英语中的连接副词有：when,where,why,how，在句中担任状语的成分。
例句：None
of
us
knows
where
these
new
parts
can
be
bought.
三.宾语从句的语序
宾语从句的语序是陈述句语序即：连接代词/副词+主语+谓语+其他成分。例句如下：
I
don’t
know
what
they
are
looking
for.
Could
you
tell
me
when
the
train
will
leave?
Can
you
imagine
what
kind
of
man
he
is?
四.宾语从句的时态
主句是一般现在时，从句根据实际情况使用任何时态。例句：The
headmaster
hopes
everything
goes
well.
主句是过去时态，从句须用过去时态的某种形式。例句：She
was
sorry
that
she
hadn’t
finished
her
work
on
time.
当宾语从句表示的是一个客观真理或者事实时，即使主句是过去时，从句也用一般现在时态。例句：The
teacher
told
his
class
that
light
travels
faster
than
sound.

"if"从句必须满足“主将从现”，而“主将从现”分为三钟情况，（1）主句满足一般将来时态（2）主句中含有情态动词（3）主句是一个祈使句。出现这三种情况，"if”从句必须用一般现在时态。