Usually you say "I divorce you" to divorce your wife. Okay, is there any other ways to say so, formal or informal, and with legal effect?

You may say whatever you desire, until you file the proper forms with the court and a judge signs them, you aren't divorced.

There are MANY ways to express to someone that you don't want to be married to them any longer, from something as simple as "It's over" to something more formal such as "I'm going to call my attorney and have you served with divorce papers as soon as possible."

Well honey, saying 'Thank You' and 'Thanks' doesn't make any difference though as they are all the same. It's only the way you say it and the tone used that will tell the difference in whether one is sincere on saying the 2 phrases. To me, it's already a habit from young that whenever someone helped me or give me a gift, I will say 'Thank You' or 'Thanks' as polite as I can to make the sender or assistant feel happy though. That may help us more as we can help each other and stay as friends too. If I were to be the sender and people were to say either 'Thank You' or 'Thanks', I will gleefully reply 'You're Welcome' or 'It's My Pleasure' so as to give people the impression that I'm someone whom people wants to meet though. I am a Singaporean who speaks English most of the time though. Furthermore, English has been Internationally recognised and Singapore is one whereas majorities speak English. Cheers and have a nice week ahead :)