Dutch adjectives change their ending depending on where they appear in the sentence and which article precedes them. The rule has two parts. First, adjectives used predicatively (after the noun, connected by a verb like zijn) never add an ending: “De auto is groot” (The car is big). Second, adjectives used attributively (directly before the noun) usually add -e: “de grote auto” (the big car).
The exception — where no -e is added — occurs with het-nouns in the indefinite singular. When you have een + adjective + het-noun, the adjective stays uninflected: “een groot huis” (a big house) — not “een grote huis.” But with het (definite): “het grote huis” (the big house) — the -e appears. This single exception is the most common source of adjective errors. A simple memory aid: een + het-noun = no -e on the adjective.
The rule holds for all adjectives in all positions, including after dit, dat, elk, ieder, welk. Adjectives after geen (no) and mijn, jouw, zijn (possessives) follow the same pattern as the indefinite article: no -e with het-nouns in the singular, -e in all other cases. Comparative and superlative adjectives also take the -e ending: “de grotere auto”, “het grootste huis.” Drill this with ten noun phrases daily until it becomes automatic.