This is what I tell my students: 0:00 - 11:59 : morning 12:00 - 17:59 : afternoon 18:00 - 20:59 : evening 21:00 - 23:59 : night However, when I'm out and it's dark, I would still say night, even though it's 1:30 in the morning.

"evening" starts around 5-6pm until the morning. "night" is whenever it's dark out. The terms overlap a lot. For expressions... "Good evening!" usually means "Hello" any time after 5pm. "Good night." usually means "Goodbye" any time when it's dark.

Its about the time. But you just can say "good night" to someone when they about to sleep.

Obviously, this varies a bit with location! To me, evening is from when it starts to get dark, until near the end of the day. This obviously varies with the season, if you are in higher latitudes. So, here in summer, evening would probably be from 8pm to 10 or 11pm. In winter, it would be maybe 5pm to 10pm.

In Britain, we only say good night before going to bed. Here we take "evening" to mean the period from 6 pm to bedtime; and "night" from sunset to sunrise the next day. "Night" only means "evening" in this kind of construction: "My parents do not allow me to go out on weekday nights."