FOR AND SINCE

1. We use for when we are talking about the duration of an action or state

+ I have lived in London for seven years.

This tells us how long I have lived in London)

NB: For is a preposition here.

2. We use since when we are talking about when the action or state started

+ I have lived in London since 1997.

This tells us when I started living in London.)

NB: Since can be a preposition (since five o'clock) or a conjunction (since I met her).