since的用法。

Question

It is three years she was in our school .
前一句是说她不在我们学校三年了。那么如果要翻译成她来我们学校三年了，要怎么说？
It is three years he stopped smoking .
It is three years he smoked .
这两句的话意思又是一样的。这个又为什么？
有说可以这样写，It is three years he have smoked.意思是不变的，
也可以这样It has been three years he smoked.是不是？

我始终不懂since在It is +一段时间+since 中的用法。一会要反过来翻译，一会又可以正常翻。请详细说一下，谢谢，

Answer 1

since
[sins]
adv.
1.
此后，自那以后；后来2.
之前，以前
prep.
1.
从…至今：例句:
there
have
been
many
changes
since
the
war.
从战争到现在发生了许多变化。2.
自从…，自…算来，自…以来：例句:
it
has
been
raining
since
morning.
从早晨到现在一直在下雨。
conj.
1.
以后，在…以后的时间里：例句:
she
has
rung
me
once
since
she
left.
她离家后给我来过一次电话。2.
[常前接
ever]
自…以来，自…至今：例句:
i've
been
busy
ever
since
i
came
back.
自我回来后一直在忙。3.
因为；既然：例句:
since
this
is
so,it's
better
to
accept
it.
既来之，则安之。
短语
1.
ever
since自…以来；一直到今2.
long
since很久以前，早已3.
since
when自什么时候起，打什么时候开始

Answer 2

since可以是介词后面加名词，一般都是过去的事，句子的谓语用现在完成时。如I
have
lived
here/have
been
living
here
since
1990.
He
has
been
reading
since
breakfast.
也可以是连词，后面接过去时的句子，主句也用现在完成时，因此我们可以说since是现在完成时的标志。如
He
has
taught
here
/has
been
teaching
here
since
he
graduated
from
colleage.
此外还有ever
since.since
then.主干都是现在完成时。

Answer 3

It has been 3 years since she was in our school.
他停止吸烟已经三年了。
他已经三年没有吸过烟了。
eg.
It’s a long time since he studied English 他好久没学英语了。
You haven’t come to see me for two months. 你有两个月没来看我了。
好像不对？
you can look up a grammer dictionary.
有人给你详解了。

Answer 4

1)
since
+过去一个时间点　(如具体的年、月、日期、钟点、1980,
last
month,
half
past
six)。
　
I
have
been
here
since
1989.
　
2)
since
+一段时间+
ago
　
I
have
been
here
since
five
months
ago.
3)
since
+从句　
　
Great
changes
have
taken
place
since
you
left.
　
Considerable
time
has
elapsed
since
we
have
been
here.
4)
It
is
+一段时间+
since从句
　
It
is
two
years
since
I
became
a
postgraduate
student.

Answer 5

1
翻译成
自从，一般是和现在完成时连用的
i
have
studied
here
since
2007
2
翻译成
既然
引导让步状语从句
since
everybody
is
here
,
let's
start
our
meeting
now.