Adjective Declension — the complete system

📌 BEFORE YOU START
Adjective declension has a reputation as the point where learners give up on German. Forty-eight cells to memorise across four cases, three genders and plural.
Forget that. There are not 48 rules. There is ONE question.
THE QUESTION THAT SOLVES EVERYTHING
"Does the article already show the gender marker?"
→ IF YES: the adjective rests. It takes only -e or -en.
→ IF NO: the adjective TAKES OVER the missing marker.
THE GENDER MARKERS
Masculine → -R (deR)
Feminine → -E (diE)
Neuter → -S (daS)
Plural → -E (diE)
NOMINATIVE
der schöne Mann (der shows -r → adj. gets -e)
ein schönER Mann (ein shows nothing → adj. takes -er)
die schöne Frau / eine schöne Frau (both show -e → adj. -e)
das schöne Kind (das shows -s → -e)
ein schönES Kind (ein shows nothing → -es)
die schönEN Kinder (plural "die" is identical to feminine "die" → -en disambiguates)
schöne Kinder (no article → adj. takes -e)
ACCUSATIVE — only masculine changes
Feminine, neuter and plural: IDENTICAL to nominative.
der → den schönen Mann
ein → einen schönen Mann
Both "den" and "einen" end in -EN — that ending transfers to the adjective.
No article: schönEN Wein
DATIVE and GENITIVE — the easy part
WITH an article: adjective always takes -en. All genders. No exceptions.
Why? Because dem, der, des, den, einem, einer, eines ALL carry the marker.
WITHOUT an article: the adjective takes the full marker.
mit schönEM Wein / mit schönER Musik / bei schönEM Wetter / mit schönEN Freunden
⚠️ The single genuine exception in the whole system: genitive masculine and neuter without an article take -en, not -es, because the noun itself already carries the -s (schönen Weines).
🧠 REMEMBER
"One marker per phrase."
Gender and case must appear ONCE — not twice, not zero times.
Article marked it → adjective rests.
Article did not → adjective works.