In short

When an adjective stands before a noun (der gute Mann), it takes an ending. Which ending depends on the article in front of it: weak after der/die/das (only -e or -en), mixed after ein/kein/mein, and strong when there is no article. Guiding idea: something must show the case - if the article doesn’t, the adjective does.

The rule

This only happens with attributive adjectives - adjectives before the noun. After sein/werden/bleiben the adjective stays endingless (Der Mann ist gut - see lesson 16). When context makes the noun obvious you may drop it, and the adjective keeps exactly the ending it would have had (Welchen Becher nimmst du, den großen oder den kleinen?) - that is still attributive, not the endingless form. There are three patterns:

1. Weak - after the definite article der/die/das and der-words (dieser, jeder, jede, alle, welcher). The article already shows gender and case, so the adjective does the least possible work: it is just -e in five “easy” spots (the whole nominative singular, plus the feminine and neuter accusative) and -en everywhere else.

der gute Mann, die gute Frau, das gute Kind - but den guten Mann and die guten Kinder.

2. Mixed - after the indefinite article ein/kein and the possessives mein, dein, sein…. These look just like the weak forms, except in the three spots where ein itself has no ending. There the adjective must jump in and carry the gender:

ein guter Mann (masc.), ein gutes Kind (neut.) - but accusative einen guten Mann, and feminine eine gute Frau.

3. Strong - when there is no article at all (often with plural, with mass nouns, or after viel, etwas, wenig). Now nothing in front shows the case, so the adjective itself takes the full der/die/das-type ending:

guter Wein, kaltes Wasser, frische Milch, dative mit gutem Wein.

Forms & patterns

Weak - after der/die/das, dieser, jeder, alle (adjective gut):

Mixed - after ein/kein/mein (adjective gut). It copies weak everywhere except three spots: nom. masc. ein guter and nom./acc. neut. ein gutes:

(ein has no plural; use kein- or a possessive: keine guten Kinder.)

Strong - no article (adjective gut). The ending copies the definite article: der->-er, das->-es, die->-e, dem->-em, den->-en:

Spelling: a stem can shrink the moment an ending is attached. hoch becomes hoh- (ein hohes Regal, never hoches), adjectives in -el drop the stem e (dunkel -> der dunkle Gang), and so do teuer and sauer (eine teure Uhr) - but sauber, bitter, munter keep it (ein sauberes Hemd).

The genitive endings (mostly -en on masc./neut.: guten Weines) come back in lesson 34; for now master nominative, accusative and dative.

Examples

Common mistakes

• ein gutes Mann -> ein guter Mann - after ein (no ending) the adjective must show masculine -er.
• den gute Mann -> den guten Mann - masculine accusative is always -en (weak and mixed).
• mit gut Wein -> mit gutem Wein - no article = strong: the adjective takes the dative -em.
• die gute Kinder -> die guten Kinder - after der-words the plural adjective is always -en.
• ein gut Kind -> ein gutes Kind - neuter after ein needs -es, because ein shows nothing.

Tips

• One question first: is there an article? No article -> strong (adjective gets the der/die/das ending). der/die/das -> weak. ein/kein/mein -> mixed.
• Weak shortcut: after der-words it is -e in five spots (the whole nominative + feminine/neuter accusative) and -en in all the rest. When in doubt, -en is the safer guess.
• Mixed = weak with three repairs: ein guter (nom. masc.), ein gutes (nom. & acc. neut.). Everywhere else, copy the weak forms.
• Strong endings are the article endings: think “the adjective becomes the article” - guter = der, kaltes = das, gutem = dem.