"I have been waiting for your reply since last 20 days" is incorrect. It should be: I have been waiting for your reply for the last 20 days. The use of the word for means you have been waiting for that length of time. We would use since differently. For example: I have been waiting since September 20. You would use for to describe the duration of the time you have been waiting. You would use since to describe when you started waiting.Originally Posted by Arun
Does that help?