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.