"I write a new letter."
Translation:J'écris une nouvelle lettre.
Some adjectives go before the noun, some after. Colour comes after; age comes before. Mostly adjectives follow the noun, and there are a few acronyms to remember the ones that precede it. BANGS (Beauty, Age, Number, Goodness, Size), for example.
Sometimes an adjective means something different depending on whether it leads or follows the noun.