if it's not so serious things just say " it's okay no worry" :)

simply you can say "ok its all right"or "oh leave it".

It depends. If the act was deliberately done to cause you pain and you were hurt it's still not fair to say "it's alright". If the person says "sorry" , you can simply say or ask "why did you do that?". Sometimes it's important to let others know how you feel.

some possible response: "that's alright" "nothing to worry about" "don't mention it" "everything is okay" if you think the person has nothing to be guilty about, you can simply say: "sorry, for what?" if the person is asking an apology for the 'major' damage he/she did, just say: "sorry? is that all you can say?" just remember though that nowadays, many people are feel sorry but few has the courage to utter the word. when someone says 'sorry' to you, just feel free to accept it by heart. :)

you may say to those who apologize to you: don't worry, no problem, It's not that big deal