一、Since 用来说明动作起始时间，for用来说明动作延续时间长度。
例如：
I have lived here for more than twenty years.
我住在这儿二十多年了。
I have lived here since I was born.
我从出生起就住在这儿了。
注意：并非有for 作为时间状语的句子都用现在完成时。
I worked here for more than twenty years.
(我现在已不在这里工作。)
I have worked here for many years.
(现在我仍在这里工作。)
注意：用句型转换的方法，很容易排除非延续动词在有for/since结构的完成时中的误用。
1)(对) Tom has studied Russian for three years. = Tom began to study Russian three years ago, and is still studying it now.
2)(错) Harry has got married for six years. = Harry began to get married six years ago, and is still getting married now.
显然，第二句不对，它应改为 Harry got married six years ago. 或 Harry has been married for six years.
二、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、1980, last month, half past six)。例如：
I have been here since 1989. 1989起，我一直在这儿。
2) since +一段时间+ ago。例如：
I have been here since five months ago. 我在这儿，已经有五个月了。
3) since +从句。例如：
Great changes have taken place since you left. 你走后，变化可大了。
4) It is +一段时间+ since从句。例如：
It is two years since I became a postgraduate student. 我考上研究生有两年了。
