Gute and Guten
I'm new, very new if I should say. But I have been very curious about something. Is there a difference between Gute (as in Gute Nacht) and Guten (as in Guten Morgen)?
Oh boy, adjective declensions! Basically when an adjective is placed before a noun, the ending is altered to complement the gender, case, and article. My favorite resource for learning this is from Nancy Thuleen's Grammar Explanations.
"Nacht" is a feminine noun, so the adjective gut only needs an "e" added to it. If there is an article (i.e. die or eine) before it, it does not change in this instance: Die gute Nacht, oder eine gute Nacht.
Adjectives also change with the case in mind (i.e. nominative, accusative, dative, genitive). The thing about the phrase Guten Morgen is that it is short for "Ich wünsche dir einen guten Morgen" (I wish you a good morning), so Morgen is a masculine noun and is in the accusative case here, thus the -en ending.
Keep in mind that "Guten Morgen" (and "Gute Nacht") is a shortened phrase, and if you simply wanted to say "a good morning" then it would be "ein guter Morgen." The -er ending is used to indicate that Morgen is masculine since ein could be neuter or masculine in the nominative case! But if the article was "der" instead, it would be "der gute Morgen", since the article already indicates the masculine noun. The -er is not needed here.
The phrase "Gute Nacht" does not have a -en ending since feminine nouns do not alter the adjective in that way, so it is "gute" for both nominative and accusative no matter the preceding article.
These are rules you will come to pick up and get the hang of eventually. Again, check out the Nancy Thuleen resource, as I think it will help you understand the existence of the rules and when to apply them.
First of all, a big welcome to you! I hope you will really enjoy learning here together with everyone!
Secondly, yes, there is a difference between "Gute" and "Guten", I'm going to explain your example: "Morgen", "Tag" and "Abend' are masculine, so you use "Guten" for them, however, "Nacht" is feminine, so you say "Gute Nacht".
Here is a pretty good explaination of the words you mentioned: https://www.duolingo.com/comment/17356895
You can also use this chart for determining endingsn (Gut-e, Gut-en etc.): https://www.duolingo.com/comment/27038339
I recommend that you write this down, print it or copy it into a notepad if it was useful, and also all other good posts or comments. That's what I do, and it's saved me lots of effort.
If you want any more help with, for instance, the genders or words, please let me/us know, I'd love to help all I can. We're all learning together! I hope this helped.
Short answer: gender & case. Gute ist feminine accusative and guten Masculine accusative. Accusative is the indiriect objective - example is 'I love him(him is accusative)' instead of 'I love he' so in the sentence 'ich wünsche dir einen guten Tag' is because der Tag (masculine)
This topic will just come to you the more you learn, just know now it is gender and case, just memorise right now the word and slowly work with the grammar. It takes time.
I've encountered these examples in an 'Intermediate German' text: Sie entwarfen immer gute Pläne. They always drew up good plans. Er hat uns keine guten Pläne gegeben. He hasn’t given us any good plans. What's the rule explaining a usage of 'gute' and 'guten' in such cases? Appreciated!