Is it true that regardless of the time of the day, the first wish to a person must be Good morning? Even if I meet him in the afternoon?
closed as not constructive by FumbleFingers, Matt Эллен, jwpat7, KitFox♦, Daniel δ Dec 20 '11 at 16:53
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No it is not.
Good night never means "hello", always "goodbye".
There are times of day when there is no really appropriate "Good ..." -- for example, Good morning is not appropriate if you join your friends in a nightclub at 12:30 am, but neither is Good evening.
Slim is entirely correct.
One occasional exception to those rules is that "Good morning" is often used ironically.
For example, if it's 4pm and I meet a coworker who looks tired, I might say "Good morning" in a joking tone, implying that he seems to have just woken up.
Or if a friend accompanied me to a late-night movie and fell asleep halfway through it, when I woke her up after the movie I might say "Good morning" as a friendly way of ribbing her for falling asleep.