if引导什么从句，遵循什么原则，哪部分是从句
if做连词，主要有两个意思，一是“是否”，引导宾语从句，一是“如果”，引导状语从句一、if作为连词，引导条件状语从句，它表示的意思是“假如”“如果”等。在复合句中如果主句用将
来时，则if引导的状语从句用一般现在时。例如:
1.
If
itdoesn’train,
we
will
go
to
the
park
next
Sunday.
如果天不下雨，下周星期天我们将去公园。
2.
If
you
ask
him，he
will
help
you.如果你求他，他将会帮助你。
if从句中用过去式，表示不可能实现，大概不会实现或提出作为考虑的假定条件。此时需要用虚拟语气，例如:
1.
If
you
were
a
bird,
you
could
fly.假使你是只鸟，你便会飞了。
2.
If
I
asked
him(if
I
were
to
ask
him)
for
a
loan，would
he
agree?如果我向他借贷，他会答应吗?
另外，if从句中用过去完成式，表示过去未实现的条件(例如由于不可能实现或某人之未能实行)。例如:
1.
If
they
had
startedearlier，they
would
have
arrivedin
time.要是他们早些动身，他们便可及时到达了。
2.
If
they
had
not
started
when
they
did，they
would
not
be
here
now.如果他们那时不动身，现在他们就不会在此地了。
二、if还可以引导让步状语从句。这时if当作“即使是”，“虽说”解。例如:
1.
If
she’s
poor，at
least
she’s
honest.
虽说她很穷，但至少她还是诚实的。
2.
If
I
am
wrong，you
are
wrong，
too.
即使说我错了，那么你也不对。
3.
I’ll
do
it,
even
if
it
takes
me
all
the
afternoon.
虽然会花费我一下午的时间，我还是要做这事。
三、if作为连词还可以引导宾语从句。引导宾语从句时和whether意思相同。例如:
1.
Lily
asked
if
/
whether
she
liked
it.
莉莉问她是否喜欢它。
2.
She
asked
if
/
whether
they
had
a
cotton
one.
她问是否他们有一件棉织的。
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