i have learned it since three years ago.这里since做介词?three years ago不是副词吗?

题目

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？

答案

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