You can say either "I haven't seen him for years" or "I haven't seen him in years," but I don't think "I haven't seen him since years" is correct English.
But you could say, "I haven't seen him since last year."
When do we use since and when do we use for in[STRIKE] the [/STRIKE]sentences?
e.g He looks different. I haven't seen him (since :cross:\ for :tick years.
Thanks.
I have not seen him since last Monday. I have not seen him for 5 minutes. I have not seen him for some days. I have not seen him for years. I have not seen him for a year. I have not seen him since last Christmas. I have not seen him since yesterday.
The difference between a time span (for) and a point of time (since):
You could raise or reduce a time span (for), examples: ...for 5 minutes -> it could also be 4 minutes, or 6 minutes.
...for some minutes -> some is also variable
...for some weeks -> it could be 2 weeks, 3 weeks or whatever
...for years -> could be 2 years, 3 years, or more, or of course also less
...for a year -> this could also vary, you could just add +1 and it would be 2 years.
You cannot do the same with since: ...since last Christmas -> this is a fix moment
...since last Monday -> a fix day
...since yesterday -> yesterday is yesterday, nothing more, nothing less.
Remember:
for: Can vary.
since: Cannot vary.
Cheers!