1. Since+时间点，具体时间
“自从……以来，从……以后” 用来说明动作起始时间
I have been in Beijing since 2010.
2. For+时间段，用来说明动作延续时间长度，因此句中的谓语动词，也应该是延续性动词。
I have been in Beijing for one year。
We have learnt English for about three years.
I have been here for 3 days. (对)
I have arrived here for 3 days.（误）
