Das Auto ist schnell.
English: The car is fast.
Adjectives in German describe people, things, or situations. The basic form of an adjective is called the 'Grundform'. This is the simple, unchanged form of the adjective, used when not adding endings.
Use the basic form of a German adjective when the adjective comes after the verb 'sein', 'werden', or 'bleiben', or when it stands alone, not before a noun.
Das Auto ist schnell.
English: The car is fast.
Die Blume ist schön.
English: The flower is beautiful.
Er ist freundlich.
English: He is friendly.
Das Wetter bleibt kalt.
English: The weather stays cold.