When do adjectives NOT decline?
Adjectives do not decline when they are used predicatively (after a form of sein, werden, or bleiben) or adverbially (modifying a verb or another adjective).
Example
Predicative: Das Auto ist rot. (The car is red.)
Adverbial: Er fährt schnell. (He drives fast.)
Adverbial (modifying adj): Sie ist sehr schön. (She is very beautiful.)