The morning in German is “der Morgen”. If you want to say “It’s a good morning”, you say “Es ist ein guter Morgen”, but if you’re wishing someone a good morning (jemandem einen guten Morgen wünschen), the morning becomes the object of the sentence, so the ending changes to “Guten Morgen”. It’s always a nice conversation starter, and you can usually get away with wishing someone a good morning until noon.

Source: Berlitz