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5. 在it is+时间+since从句结构中，since从句是持续性动词的一般过去时，含有否定的意味。
例如：
It is three years since she stayed here.
她离开这里已经3年了。
How long is it since you were a league member?
你_有多久了?
6. 在it is+时间+since从句结构中，since从句是现在完成时或过去完成时，均含有否定意味。
例如：
It is five years since we have seen her.
我们已经5年没有见她了。
It was years since I had seen her.
在那之前，我已经有好久没有见到她了。
It’s been a long time since I’ve seen you.
我已经很久没有见到你了。

另外，①依据英语惯用法，since从句不行以否定形式出现。
例如：
It is four years since I have smoked a cigarette. 我已经4年不抽烟了。
不行说：It is four years since I haven’t smoked a cigarette.
但由于下句中的since不作“自从”解，所以从句可以用否定形式。
例如：
I haven’t been out anywhere since I don’t know how long.
也不知多久了，我哪儿也没去过。
②Since从句通常不与非持续性动词的现在完成时的确定式连用。
例如：
Her mother died when she was a child.
她小时候母亲就去世了。
不行说：Her mother has died since she was a child.
③Since前不行加表示时间的词语，但可以与ago连用。
例如：
She has lived with us since her mother died three years ago.
自从她母亲3年前去世，她就始终和我们住在一起。
不行说：She has lived with us three years since her mother died.

④在It was five years ago since his father died. 一句中的since用错了，因为在含有ago的时间范畴里，已经有了从过去的一个时间点以来的时间，即已有了since的时间范畴，所以再用since是多余的。
改为：
It was five years ago that his father died.
他父亲5年前去世了。
It is five years since his father died.
他父亲去世已经5年了。
高三学年英语必修书的必拿下学问点2

主语从句是在复合句中充当主语的从句，通常放在主句谓语动词之前或由形式主语it代替，而本身放在句子末尾。
1. It 作形式主语和it引导强调句的比较。
It 作形式主语代替主语从句，主要是为