第2个回答 2020-02-21
1.
当if后面接一般现在时的动词时,表示对将来的一种推测.
if
sb
do
sth
sb
will/shall
do
sth.
if
you
go
with
me,
you
shall
be
rewarded.
2.
当if后面接一般过去时的动词时,则是一种虚拟语气.
if
sb
did
sth
sb
would/should
do
sth.
if
i
didn't
make
that
mistake,
i
would't
have
suffered
such
a
failure.
3.
当if后面接什么时态都可以时，if为“是否”的意思，句子为宾语从句.
sb
do
sth
if
sb
do
sth.
he
said
if
rain
was
stop.
由if引导的条件状语从句，时态应该是主将从现，就是说主句用一般将来时，而从句用一般现在时表示将来。
例如
if
it
doesn't
rain
tomorrow,
i
will
visit
my
former
school.
这里从句就用一般现在时。
当然if引导的从句也可以用虚拟语气
例如if
i
got
up
early
in
the
morning,
i
wouldn't
miss
the
train.
第3个回答 2019-05-03
if引导的条件句中的虚拟语气
if引导的条件状语从句可以分为两类：真实条件句（Sentences
of
Real
Condition）和虚拟条件句。凡是假设的情况发生性可能很大，就是真实条件句；当假设是不大可能实现时，就是虚拟条件句。如：
If
I
have
time,
I
will
help
you
with
this
work.
如果我有时间,
我会帮助你做的。
If
time
permits,
we'll
go
fishing
together.
如果有时间的话，我们就一起去钓鱼。
If
I
were
you
,
I
would
have
attended
the
meeting.我要是你的话，就去参加会议了。
If
he
had
come
here
yesterday,
he
would
have
seen
his
old
friend.假如他昨天来这儿的话，就会看见他的老朋友。
下面我们一起看看if在条件状语从句中的虚拟语气。
1.
表示与现在事实相反的情况。其句子结构为：从句：if
+
主语+
动词的过去式(be
用were)
+
……主句：主语+
would
(should,
could
,
might)
+
动词原形+
……
If
I
were
you,
I
would
read
it
again.
如果我是你的话，我再读一遍。（事实上我不是你）
If
he
hurried,
he
could
catch
the
first
bus.
他要是快点能够赶上头班公共汽车。（可他不急）
If
she
knew
English,
she
would
not
ask
me
for
help.
如果她懂英文，她就不必要我帮了。
2.
表示与过去事实相反的虚拟条件句。其句子结构为：从句：If
+
主语+
had
+过去完成式动词+
……主句：主语+
would
(should,
could,
might)
+
have
+过去完成式动词+……
If
you
had
taken
my
advice,
you
would
not
have
made
such
a
mistake.如果你听了我的劝告,就不会犯这样的错误。（事实上你没有听我的劝告）
If
it
had
rained
yesterday,
we
would
have
stayed
at
home.
如果昨天下雨的话，我们就会留在家里。
If
I
hadn’t
been
ill
yesterday,
I
might
have
come
to
school.昨天要是不生病，我是可能来上学的。（事实上我生病了）
If
you
had
studied
harder
last
term,
you
could
have
passed
the
exam.
如果你在上个学期用功一些，你就会在考试中过关了。
3.
表示与将来事实相反的虚拟条件句
(对将来的事实实现的可能性不大)。其句子结构为：从句：If
+
主语+
should
(或were)
+
动词原形+……主句：主语+
would
(could,
should,
might)
+
动词原形+……
If
it
should
rain
tomorrow,
I
would
stay
at
home.
如果明天下雨的话，我将待在家里。（根据天气情况，明天不可能下雨）
If
he
came
tomorrow,
I
would
do
it
with
him.
如果明天他来的话，我将和他一起做此事。（事实上他来的可能性很小）
If
he
were
here,
I
would
give
him
the
books.
如果他在这儿，我可能会把书给他。