Why we say 'guten Tag': greetings in the accusative case
When you first learn German, you quickly pick up everyday phrases like guten Tag, guten Morgen, or guten Abend. If you look closely at the adjectives in these greetings, you might notice that they have specific endings, like the -en in guten. Why do we not just use the basic dictionary form and say guter Tag?
The answer lies in a hidden grammatical structure. Common greetings and wishes in German are actually shortened versions of a full sentence. English speakers often try to analyze these phrases on their own, but to understand the endings, you have to look at the invisible words that were left out.
When you wish someone a good morning, the complete thought is ich wünsche dir einen guten Morgen (I wish you a good morning). Because the greeting is the direct object of the verb, the entire phrase is in the accusative case. Over time, the German language dropped the first half of the sentence, but the accusative adjective ending remained.
This is most obvious with masculine nouns, which take a clear -en ending in the accusative case. This is why you will hear guten Tag and guten Appetit instead of the nominative forms.
This rule also applies to expressing gratitude and offering praise. When using masculine nouns for thanks or praise, you will see the exact same accusative ending. The hidden sentence is usually something like ich sage dir vielen Dank (I say many thanks to you) or ich wünsche dir herzlichen Glückwunsch (I wish you sincere congratulations).
You will use these endings for phrases like vielen Dank and herzlichen Dank (thank you very much) as well as the plural herzliche Glückwünsche. For birthdays, you can say herzlichen Glückwunsch zum Geburtstag or alles Gute zum Geburtstag, which both act as accusative objects of a hidden sentence.
Feminine nouns, plural nouns, and uncountable nouns also sit in the accusative case in these greetings, even though their endings do not look different from the basic nominative case. For example, gute Nacht, gute Fahrt, and gute Reise all use feminine nouns. Phrases for the holidays like frohe Weihnachten or fröhliche Weihnachten are plural.
Finally, expressions with uncountable concepts like viel Glück or viel Erfolg function exactly the same way. You can always assume they are the direct object of a hidden wish.
When you learn a new greeting or wish in German, remember that it is usually the direct object of a hidden verb like wünschen (to wish). The adjective endings are locked into the accusative case. Learning these fixed phrases as complete chunks will make your German sound highly natural.