I posted the letter. I realized that I had forgotten to put on a stamp.

Only after posting the letter did I realize that I had forgotten to put on a stamp.

Hardly had I realized that I had forgotten to put on a stamp when I posted the letter.

Not only did I realize that I had forgotten to put a stamp on the letter but I posted it as well.

No sooner had I posted the letter than I forgot to put on a stamp.