How do German adjective endings work?
An adjective before a noun takes an ending; an adjective after sein or werden takes none at all. The ending depends on whether the article already shows the case and gender. If it does, the adjective takes a weak ending — mostly -e or -en. If there is no article, or the article is uninformative, the adjective has to carry the information itself.
Only adjectives before nouns take endings
This halves the problem immediately and is the first thing to internalise. Predicate adjectives — after sein, werden, bleiben — never change, regardless of gender, number or case.
Das Auto ist neu.
The car is new.
After sein — no ending.
Das neue Auto.
The new car.
Before the noun — ending required.
The article decides how much work the adjective does
The underlying logic is that case and gender must be marked once. If a definite article already shows it, the adjective can relax into a weak ending. If there is no article at all, the adjective takes the strong ending the article would have had.
der gute Wein
the good wine
Definite article — weak ending.
ein guter Wein
a good wine
Ein shows nothing, so the adjective carries -er.
guter Wein
good wine
No article — the adjective does all the work.
A practical shortcut
Rather than the three full tables, most sentences are covered by two observations. In the nominative and accusative singular after a definite article, the ending is -e, except masculine accusative which is -en. Everywhere else after a definite article — all plurals, all datives, all genitives — it is -en.
der gute Wein / den guten Wein
nominative / accusative masculine
die gute Frau / die gute Frau
feminine stays -e in both
mit dem guten Wein
dative — always -en
die guten Weine
plural — always -en
Common mistakes
Das Auto ist neues.
Das Auto ist neu.
Predicate adjectives take no ending. The adjective only inflects when it sits before the noun.
ein gute Wein
ein guter Wein
Ein does not show masculine nominative, so the adjective must — hence -er.
mit dem gute Wein
mit dem guten Wein
Everything in the dative takes -en after a definite article.
Frequently asked questions
Do I need to learn all three tables?
Eventually you will absorb them, but starting there rarely works. The two observations above cover the large majority of real sentences, and the rest is picked up from reading rather than recited.
What happens with kein and the possessives?
They behave like ein — they show some information but not all — so adjectives after them follow the mixed pattern rather than the definite-article one. Mein guter Freund, not mein gute Freund.
Does this matter if I just want to be understood?
You will be understood with wrong endings, and Germans will not stop you. But endings are the single clearest marker of whether someone has read much German, so they are worth getting right eventually.