What is the difference between following two sentences?
1 : I'm sorry to keep you waiting.
2 : I'm sorry to have kept you waiting.
I guess that I should use the first when someone will wait for me from now on, and I should use the socond when someone has been waiting for me until now...................
Thank you for your time.
Generally speaking, that is correct. However, I'd explain it like this.
1. Use 1 if the person has to continue waiting.This would most likely be followed by a reason that the person has to continue to wait. At the moment that the person no longer has to wait, some people might use 1 also.
2. Use 2 when someone no longer has to wait. You should use sentence number 2 at the moment that the person no longer has to wait. That sentence would not work very well if too much time has passed. It is best used at the time or moment that the person no longer has to wait.