Adjektive.
How they describe nouns.

Adjectives describe nouns: big, small, old, new, interesting, difficult. In German, where an adjective sits in the sentence determines whether it changes form or not. After a verb like sein it stays as-is. Before a noun, it takes an ending. That ending depends on the case, gender, and article of the noun it describes.

Two positions, two behaviors

The first thing to understand about German adjectives is that position decides everything. An adjective after sein or werden is called prädikativ: it describes the subject and never changes form. An adjective directly before a noun is called attributiv. It takes an ending every time.

Position	Name	Example	Adjective changes?
After sein / werden	Prädikativ	DasBuch ist interessant.	No, stays as-is
Before the noun	Attributiv	DasinteressanteBuch.	Yes, takes ending

Prädikativ: adjective after sein or werden

When an adjective follows sein, werden, or bleiben, it describes the subject but does not agree with it. The form is always the base form: no endings, no changes.

• DasWetter ist schön. (The weather is beautiful.)
• DerKaffee ist kalt. (The coffee is cold.)
• DieAufgabe ist schwierig. (The task is difficult.)
• Es wird dunkel. (It is getting dark.)

The adjective is the same regardless of whether the subject is masculine, feminine, or neuter. That is the advantage of the prädikativ position: no endings to worry about.

Attributiv: adjective before the noun

When the adjective stands directly before the noun, it must agree with that noun. The ending signals the gender and case of the noun. This is called Adjektivdeklination (adjective declension).

The ending the adjective takes depends on three things: which case the noun is in, the gender of the noun, and which article (if any) precedes the adjective. With a definite article → der die das, the endings are mostly -e or -en.

Case	Maskulinum	Femininum	Neutrum
Nominativ	deralteMann	diealteFrau	dasalteKind
Akkusativ	denaltenMann	diealteFrau	dasalteKind
Dativ	demaltenMann	deraltenFrau	demaltenKind
Genitiv	desaltenMannes	deraltenFrau	desaltenKindes

The pattern with a definite article is the most regular → Nominativ singular uses -e throughout, and most other forms use -en. The Akkusativ masculine is the one exception in the singular → -en because the article already changes to den.

The logic behind the endings

Adjective endings are not random. They follow a clear principle: somewhere in the noun phrase, the gender and case must be visible. The article shows this most of the time. When the article already carries a strong signal, like der, den, dem, the adjective ending can be weak (-e or -en). When the article is missing or gives no signal, the adjective must step in and carry the ending itself.

• DergroßeHund schläft. (The big dog is sleeping.) article signals masculine Nominativ, adjective uses -e
• EingroßerHund schläft. (A big dog is sleeping.) indefinite article gives no signal, adjective uses -er to show masculine Nominativ

This is the core idea. The adjective and the article work together. One of them always shows the case and gender. Which one does the work depends on which article is present.

Same word, no ending: Adverbien. They never change. How an adjective looks once it becomes an adverb. Also: ordinal numbers like erste and zweite take exactly these endings, see Numerale. The words that count.