BASICS
Which adjective-ending you use depends on whether or not you find an article-ending in front of the adjective.
E.g. in the sentence:
- Ich sehe einen grünen Frosch.
you find "einen" with the ending "-en" in front of grünen.
In the sentences:
- Ich sehe [X] ein grünes Auto.
- Ich sehe [X] grüne Autos.
you don't find any article-endings [indicated by the X]
NEXT STEPS
Once you have figured out whether or not there's an article-ending in front of the adjective, you "only" need to apply the following rules:
IN CASE OF AN ENDING
SIMPLY APPLY THE FOLLOWING RULES:
- Once -en always -en
- Adjectives in Plural Dative or Genitive always -en
- After der / das / die always -e
EXAMPLES FOR EACH RULE:
Ich sehe einen grünen Frosch [einen ends in -en so grünen has to end in -en as well.]
Plural: Ich sehe die grünen Frösche.
Dativ: Ich helfe dem grünen Frosch.
Genitiv: Ich esse die Beine des grünen Frosches.
(der = masculine): Der grüne Frosch ist lecker (+).
(das = neuter): Das grüne Auto ist schnell (+).
(die = feminine): Die grüne Blume ist schön (+).
(+) Adjectives at the end of a sentence never get any ending.
IN CASE THERE'S NO ARTICLE ENDING
"simply" add the ending that a regular article would take if it was in the adjectives place. A couple of examples:
- Ich sehe [X] grüne Autos. <-- grün gets the same ending as a "die" would get in its place
- Ich habe ein grünes Auto. <-- grün gets the same ending as a "das" would get in its place
This is just a rough overview of a really logical and beautiful technique which is much better explained and visualized in a video I made for those interested.
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