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.
