In English and in German, adjectives can be placed before nouns or after nouns. In German, if an adjective is placed before the noun, it takes specific endings depending on the case and gender of the noun. If an adjective is placed before the noun, and is preceded by a “der”-word (definite article, demonstrative pronouns, indefinite pronouns), then it has “weak” endings.
Example:
Der Apfel ist rot. The apple is red.
The adjective “red” is placed behind the noun “Apfel”, so the adjective has no endings, and stays the same for any noun regardless of gender. However:

Der rote Apfel schmeckt lecker. The red apple tastes delicious.
The adjective “red” is placed before the noun “Apfel” so it has an ending. Because we used the definite article “der”, the ending is considered “weak”.

In the above example “Der rote Apfel schmeckt lecker.”, the noun “Apfel” is a masculine noun in the nominative case, so the weak adjective ending is “-e” (rote).

Examples:
Die Kaffee schmeckt bitter.   The coffee tastes bitter.
The adjective “bitter” is behind the noun “Kaffee”, so it it has no ending.

Ich trinke den bitteren Kaffee.   I drink the bitter coffee.
“bitter” is before the accusative masculine noun “Kaffee”, to it has the ending “-en”.

Die Email ist lang.   The email is long.
“lang” is behind “Email”, so no ending is needed.
Er liest die lange Email. He reads the long email.
“lang” is before the accusative feminine noun “Email”, so it has the ending “-e”

HINT: All adjective endings after all der- or ein-Words in the dative and genitive cases are always ‘-en.’ This means that you only have to memorize the changes between the definite and indefinite articles tables, specifically the masculine and neuter in the nominative (+er, +es after the indefinite article), and the neuter in the accusative (+es; the feminine in both nominative and accusative in either table is exactly the same: ‘e’).

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