Question

现在完成时中since与before的用法举例

eg：1.It has been 3 years 【since】 I wrote to him.我给他写信已经3年了。
2.It has been 3 years 【before】 I wrote to him.我已经3年没有给他写信了。
我只知道些大概，希望有懂的大佬能详细地举例子讲一下这两者的用法。不用急着回答，我会仔细看，优质答案一定能被采纳！

Answer 1

for+时间段。

He has been in the army for three years.

since+时间点或句子。

He has been in the army since three years ago.

They haven't seen each other since he left three years ago.

追问

请仔细看问题，谢谢！相信你能作出更完美的回答！

Answer 2

句子1意思是我已经三年没跟他写信了，现在可能也没有写，但是只是想起了这件事情
句子2是我三年跟他没写信了，但是我现在在写，写的时候感叹了一下，都过去三年了