7. Adjective endings: signal once

A noun phrase must show its gender and case once, with a strong ending from section 5. If the word before the adjective already shows it, the adjective takes a plain weak ending (-e or -en); if nothing before it shows it, the adjective carries the strong ending itself. That is the whole system; the “three tables” in textbooks are this one rule applied three times.

The weak endings: only -e or -en

Weak endings have two forms. -e appears in exactly five cells: Nominativ singular of all three genders, and Akkusativ singular feminine and neuter. -en appears everywhere else: masculine Akkusativ, every Dativ, every Genitiv, every plural.

Case	Masculine	Feminine	Neuter	Plural
Nominativ	-e	-e	-e	-en
Akkusativ	-en	-e	-e	-en
Dativ	-en	-en	-en	-en
Genitiv	-en	-en	-en	-en

The -e cells form an L shape in the top left. The only surprise is masculine Akkusativ, and that is the same masculine-only change you saw for der → den.

The three situations

1. After a der-word (der, dieser, jeder, welcher, alle): the article always carries the strong ending, so the adjective is always weak. der alte Mann, den alten Mann, dem alten Mann, die alten Leute.
2. After nothing (no article, or an uninflected word like zwei, drei, viel, etwas): the adjective must carry the strong ending. alter Wein, frische Milch, kaltes Wasser, mit kaltem Wasser, gute Ideen. One exception: masculine and neuter Genitiv take -en, because the noun’s own -s already signals the case (trotz schlechten Wetters).
3. After an ein-word (ein, kein, mein …): the ein-word carries the strong ending in every cell except its three gaps. In the gaps the adjective takes the strong ending; everywhere else it is weak. ein alter Mann, ein altes Haus (gaps), but einen alten Mann, einem alten Haus, meine alten Freunde (article shows the case).

One noun phrase, three situations

Cell	After der-word	After ein-word	No article
Masculine Nominativ	der alte Wein	ein alter Wein	alter Wein
Neuter Nominativ / Akkusativ	das alte Bier	ein altes Bier	altes Bier
Masculine Akkusativ	den alten Wein	einen alten Wein	alten Wein
Masculine / neuter Dativ	dem alten Wein	einem alten Wein	altem Wein
Feminine Dativ	der alten Stadt	einer alten Stadt	in alter Zeit
Plural Nominativ	die alten Häuser	keine alten Häuser	alte Häuser

Bold endings are strong endings sitting on the adjective because nothing in front of it carries one.

The algorithm

1. Find the noun and settle its gender, number and case (sections 3, 4, 6).
2. Look at the word right before the adjective(s). Does it show the strong ending for that cell? der, den, dem, des, die, das, einem, einen, meiner do; ein in a gap does not; no word does not.
3. If yes, use the weak ending: -e in the five L-shaped cells, -en everywhere else.
4. If no, put the strong ending on the adjective (r, s, m, n, e from section 5), except -en in masculine and neuter Genitiv.

Details that follow from the same rule

• Several adjectives in a row share the same ending: ein großer, alter, schöner Baum; mit frischem, heißem Kaffee.
• Predicate adjectives take no ending: Der Kaffee ist heiß. Only adjectives in front of a noun decline.
• alle and beide behave like der-words (alle neuen Kollegen); viele, einige, mehrere, wenige do not (viele neue Kollegen).
• Comparatives, superlatives and participles decline the same way: ein besseres Angebot, der schnellste Weg, das trainierte Modell, eine laufende Analyse.
• Adjectives used as nouns keep their adjective endings: der Deutsche, ein Deutscher, die Angestellten; after etwas, nichts, viel they take the strong neuter ending (etwas Neues, nichts Gutes), after alles the weak one (alles Gute).

How to make it automatic

For two weeks, label every noun phrase in one short text a day: mit dem (Dat, m, strong) neuen (weak -en) Kollegen (weak masculine -n). Then reverse it: write ten phrases a day, each with a different cell, and say them aloud. Speed comes from repeating whole chunks (mit meinem besten Freund, in einer großen Stadt) until you stop computing them.