how to search mails in gmail which i delete forever and i can’t undo it ,,b’cos its too late,,please help me?

In interpreting your question, I think you want to find old, deleted emails. The problem is this. "forever" actually means forever. Don’t be afraid to click on the the "Help" link in Gmail. Gmail’s help pages are excellent. Here’s what I found:

http://mail.google.com/support/bin/answer.py?hl=en&answer=78353