An adjective can directly precede the noun it describes. In this case, the adjectival ending changes to match that of the noun it precedes. This is called adjective declension. There are three different declensions. The declension to use depends on the article that precedes the noun: the indefinite article, the definite article or no article.

Adjective declension after a definite article

Inge speaks with Nico who is wearing an old-fashioned shirt and sweater vest. — null DW

Inge schenkt Nico ein gestreiftes Hemd. – Nico findet das gestreifte Hemd nicht so schön.

If there is an indefinite article before the noun, then the adjective (in the nominative and accusative cases) takes the ending of the definite article.

But when there is a definite article before the noun, the adjective only takes the ending -e or -en.

Masculine:
der schicke Pullover (nominative)
den schicken Pullover (accusative)
Feminine:
die schicke Hose (nominative and accusative)
Neuter:
das schicke Hemd (nominative and accusative)
Plural:
die schicken Schuhe (nominative and accusative)

In the accusative case, the adjective before a masculine noun takes the ending -en.
Adjectives before plural nouns also take the ending -en.

Adjective declension after the possessive determiner or the negative article

Adjectives before a noun with a possessive determiner (i.e., mein, dein, sein …) or a negative article (kein) are declined as follows:

in the singular, the same as the adjective declension after an indefinite article.
in the plural, the same as the adjective declension after a definite article.

Masculine:
kein schicker Pullover (nominative)
keinen schicken Pullover (accusative)
Feminine:
keine schicke Hose (nominative and accusative)
Neuter:
kein schickes Hemd (nominative and accusative)
Plural:
keine schicken Schuhe (nominative and accusative)

Grammatical terms in German:

der bestimmte Artikel: The articles der, die, das are definite articles.

der unbestimmte Artikel: The articles ein, eine are indefinite articles.