Grammar

Language

Hajimemashite is used only when you meet someone for the first time.
Oyasuminasai is used when going to bed but not when leaving a friend's house at night.
Just as in English, there are different ways of saying "thank you": arigatoo (thanks), arigatoogozaimasu (thank you), and domo arigatoogozaimasu (thank you very much).
Ittekimasu is what you say when you leave the house for school. It literally means "I'm going out but will be back." When you get home, you say tadaima, which means "I've returned."