Good night and Good evening are both formal ways of greeting or saying goodbye to someone. Most of the time we just say hi or hello if it's someone we don't know that well or at all. If it is a good friend or a family member it becomes even less formal. "Hey, what's up? or "What's goin' on" are examples of greetings among friends. If you are leaving, then "See ya'" or even just "Later, dude." are common goodbyes among friends. Don't worry about it too much as you will understand this very easily over time. Very good question.

Excellent question! To be honest, most of the time we greet each other in America we either say 'Good morning' or just plain old 'Hello.' Or 'hey' is fine as well. 'Good afternoon' and 'good evening' are both very formal, you wouldn't really use them with friends or family. If you're working in customer service and speaking to a customer, however, you could use them to sound polite. You would most likely say, 'Hello ma'am/sir, how may I help you this afternoon/evening?' or some variant of that. If you have any further questions about this, feel free to ask.