since用来说明动作起始时间，而 for用来说明动作延续时间长度。

a） I have lived here for more than twenty years.

我在这住了超过 20年了。

b） I have lived here since I was born..

我从出生就在这住了。

【典型例题】

My aunt has worked in a clinic___ 1949.

A since B for

答案为： A

2) My brother has been in the Youth League___ two years.

A for B since

答案为： A

3) Some new oilfields have been opened up___ 1976.

A for B since

答案为： B

4） I have not heard from my uncle___ a long time.

A since B for

答案为： B

注意：并非有 for作为时间状语的句子都用现在完成时。如：

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。

评：虽然 since和 for都可以表示原因，但是两个词后一个接时间点，一个接时间段，请大家用心记住！