i have learned it since three years ago.这里since做介词?three years ago不是副词吗?很疑惑 如果是for three years.这里 three years是名词?另问一个问题 young sister和younger sister有什么区别？可以用young sister 表达

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i have learned it since three years ago.这里since做介词?three years ago不是副词吗?
很疑惑 如果是for three years.这里 three years是名词?
另问一个问题 young sister和younger sister有什么区别？可以用young sister 表达妹妹吗？
抱歉再补充一句，那since作介词的话后面跟名词，那时间名词是怎样？怎么区分时间名词还是时间副词？

这里的since是连接词
conj.（连接词）
During the period subsequent to the time when:
在…以后的一段时间里：
He hasn't been home since he graduated.
他毕业后没回过家
Continuously from the time when:
从…时起一直：
They have been friends ever since they were in grade school.
他们从小学起就一直是好朋友
three years ago 是时间状语
妹妹一般是younger sister
姐姐 elder sister
只说sister 表示姐妹 既可以是姐姐也可以是妹妹

since 的用法参照一下解释：
1.作为连词的since, 其意义有两个：
1） after the last time when…自从…以来，以后
2） continuously from the last time when…从…以后，就一直…
这是我们需要弄明白的第二个疑惑点。
2.在since引导的状语从句...

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since 的用法参照一下解释：
1.作为连词的since, 其意义有两个：
1） after the last time when…自从…以来，以后
2） continuously from the last time when…从…以后，就一直…
这是我们需要弄明白的第二个疑惑点。
2.在since引导的状语从句，其谓语可以是一般过去时，也可以是现在完成时。但，其谓语动词是短暂动词还是延续动词或状态动词，其表达的意思就完全不一样了。
这是正确理解since从句的关键。
3.Since从句中的谓语动词是短暂性动词：用一般过去时或现在完成时，意义一样，都表示过去时间的起点：
It is three years since I saw Bill.
It has been three years since I saw Bill.
It is three years since I have seen Bill.
句意是：从我那时看见Bill到现在已经三年了。
4.Since 从句中的谓语动词是延续性动词或状态动词，过去时态表示动作或状态的结束，现在完成时表示动作或状态的开始并延续至今：
I haven’t heard from him since he lived in London.
(他已不在London，从他离开London之时起到现在我就没有收到过他的来信)
I haven’t heard from him since he has lived in London.
(他还在London,从他搬到London起到现在我就没有收到过他的来信)
They have had three headmasters since I was at this school.
(我已不在这所学校，从我离开这所学校他们已经换了三位校长)
They have had three headmasters since I have been at this school.
(我还在这所学校，从我到这所学校起他们已经换了三位校长)
It has been three years since he lived here.（他不在这儿住了，已经三年了。）
可以用young sister 表达妹妹

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