He said,'I have been here since two years ago'把这个句子变成间接引语怎么变?是不是要把since two years ago变成since two years before?

现在完成时VS. 一般过去时
1） 过去时表示过去某时发生的动作或单纯叙述过去的事情，强调动作；
现在完成时为过去发生的，强调过去的事情对现在的影响，强调的是影响。
I lost my cell phone yesterday. / Oh no! I have lost my cell phone!
2）过去时常与具体的时间状语连用，而现在完成时通常与模糊的时间状语连用，或无时间状语。
＊一般过去时的时间状语:
yesterday, last week，…ago, in1980, in October, just now(具体明确的过去时间状语)
＊现在完成时的时间状语
for, since, so far, ever, never, just, yet, till / until, up to now, in past years, always,
(不确定、模糊的时间状语)
◇Since的四种用法:
1) since +过去一个时间点(如具体的年、月、日期、钟点) I have been here since 1989.
2) since +一段时间+ ago I have been here since two years ago.两年前我就来到了这里.
3) since +从句 Great changes have taken place here since you left.
4) It is +一段时间+ since从句 It is two years since I became a college student

since two years ago 才是正确的。He said that he hadn't been here since two years ago.

He said he hadn't been here since two year before.