Since, for, ago and before
Since, when used with the present perfect tense, means from a point or period of time in the past up to now.
- I have lived here since my childhood.
- I haven’t seen him since last week.
- India has made tremendous progress since the dawn of independence.
For, when used with the present perfect tense, refers to a period of time up to now.
- We have lived here for twelve years.
- He has been working here for three years now.
- The strike has continued for two weeks.
Ago is used when you date back from now to a point of time in the past.
- It was two years ago that I first met Alice.
- I saw him two months ago.
Before is used when you date back from any point of time in the past or future.
- The roof must be repaired before the rain starts.
- I think I have seen him before.