what to say....?
If I met someone, whom I previously knew was sick, what would be the best expression to show him/her that I was sorry?

Please, provide as many expressions as you can.Can I say,

I'm sorry for your pain.

"I was sorry to hear that you were sick. How are you doing now?"

Sorry for what? If someone's not sick any more, we don't usually mention it. However, here are a few things:

I heard you had a bad cold. Glad to hear you're much better now.
...or similar.