Saying "I seen" sounds like you are missing the helping verb (have), which you are. "I have seen" is the present perfect expression, while "I had seen" is the past perfect.

I'm guessing E will explain it better.
Originally Posted by kayb
I could explain it in a more jargony way, but that would be a worse explanation in this context.

I would point out, though, that whether "I seen" is missing the helper verb really just depends on which English you're talking about. It's like talking about whether swimsuits are appropriate. They're neither appropriate nor inappropriate per se. It depends on the situation.