Since versus For
It is used when we measure the duration, when we say how long something lasts.
For + a period of time: To measure a period of time up to the present, we use the present perfect tense and not the present tense.
- I have known her for a long time.
The present tense with ‘For’ refers to a period of time that extends into the future.
- How long are you here for?
It gives the starting point of actions, events or states. It refers to when things began.
Since + a point in time (in the past), until now.
- I’ve been waiting since 7 o’clock.
With since we use the present perfect tense or the past perfect tense.
- I have been here since 5 o’clock and I am getting tired.
Since can also be used in the structure It has been + period of time + since.
- It has been two months since I last saw her.