Americans like to say "Thank you" when others help them or say something kind to them. People of many countries do so, too. It's a very good habit.
You should say "Thank you" when someone passes you the salt on the table, when someone walking in front of you keeps the door open for you, when someone says you have done your work well, or you have bought a nice thing, or your city is very beautiful. "Thank you" is used not only between friends, but also between parents and children, brothers and sisters.
"Excuse me" is another short sentence they often use. When you hear someone say it behind you, you know that he wants to walk past you without touching you. It's not polite to interrupt (打断) others when they are talking. If you want to speak to one of them, please say "Excuse me" first, and then begin talking. You should also do so when you begin to cough or make any noise before others. Let's learn to say "Thank you" and "Excuse me".
