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？
问题描述：
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？
答
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