since和for的用法

since 用来说明动作起始时间，其后应该接一个过去的具体时间、时间段+ago或者是一个一般过去时态的从句。 例如：
　　My father has worked in the factory since 1978.
　　I have lived here since ten years ago.
　　I have known Li Ming since he was a little boy.
　　
　　for用来说明动作延续时间长度，其后接时间段。例如：
　　I have lived here for twenty-one years.
　　He has writing for three hours.
　　I have not heard from my uncle for half a year.

I haven't heard from him since last year.我自去年以来未曾收到过他的信。
I have worked here for many years. (现在我仍在这里工作。)
这应该在英语那边问啊