In German, when an adjective comes before the noun it describes, we have to change its ending. The technical term for this process is "declining" the adjective.
The adjective ending depends on the noun it's describing. More specifically, it is determined by:
The definite article already tells you the case the noun is in. That means the adjective doesn’t have to give you any extra information. Nevertheless, it still gets an ending:
⇒ Accusative masculine, all dative, all genitive, and all plural get the ending "-en."
⇒ The rest get the ending "-e."
Definite Article Words
There are other words that we treat like definite articles when determining the adjective endings. Here they are:
When an adjective comes after one of these articles, it gets the same ending as it would if there was a definite article. Because these words already have a declined ending which tells you the case, the adjectives just needs an "-e" or "-en" ending.
But those also have a plural form, so we have to fill in the bottom row of the table!
The colors show you how we know what case is being used. For plural, dative, and genitive that's the article. That means the adjective doesn't need a different ending (and we can just use "-en"). Since nominative and accusative also have articles without an ending (ein, mein,...), the adjective has to show the case. That means in nominative and accusative singular, the ending of the corresponding definite article (der, die ,das...) is added to the end of the adjective.
When a noun doesn't have any article (no "der," "mein," "kein," etc.), we say the noun is using the zero article. Here are the adjective endings we use with the zero article:
Except for genitive masculine and genitive neuter, all adjectives get the ending of the corresponding definite article! There is nothing else that indicates the case, which means the adjective must show the case.
In genitive masculine and genitive neuter, we have to add an "-s/-es" at the end of the noun. That indicates the genitive case, and therefore the adjective keeps the "en"-ending.
Examples of adjective declension after the zero article:
If an adjective ends in "-e," we don‘t add a second "e":
(Wrong: ein leisees Kind)
If an adjective ends in "-el," remove the "e" from the "el":
(Wrong: „ein sensibeles Kind“)
(Wrong: „ein dunkeler Wald“)
If an adjective ends in "-er" and has a vowel (a, e, i, o, u) directly before that "er," remove the "e" from the "-er":
BUT: „sauber“ – „ein sauberes Zimmer“ (no vowel before the "er") // "a clean room"
For the adjective "hoch," remove the "c":
For adjectives that end in "-a," as well as those that come from city names and end in "-er," there is no declension (no adjective ending):
In German, it's important to know what case every noun is in. If the article doesn't tell you what case it is, the adjective has to do that job.
In practice, that means the adjective gets the ending of the corresponding definite article for that gender and case (der, die, das,... ).
A simple step-by-step guide for finding the right adjective ending: Adjective Declension: 4-Step Rule.
An overview of what you have to pay attention to when dealing with adjectives in general.
Adjective declension depends on whether there is a definite article, indefinite article (including possessive determiners or the negative article "kein"), or no article at all (the zero article) in front of the adjective.
You have to know the gender of the noun.