Normally, "ago" refers to a completed time - it's in the past. Since, on the other hand, refers to a time when something began and continued up to another point. I came here a year ago. (completed time/action) I've been here since 3pm. (incomplete time action) You might see : I've known him since about a year ago. I've known him since February, 2005. 'A year ago' identifies a point in the past, in the same way that I might say 'February 2005'. How long have you been writing? I have been writing ince about a minute ago. Translation: I have been writing for about one minute from 1 minute which happened in the past. Ago is a synonom to say "happened in the past"