L’adjectif attributif est invariable en genre et nombre et se met toujours devant le substantif au qu’il se réfère.
That is a famous writer. Those are famous writers.
She's a pretty girl. They are a pretty girls.
This is an old house. These are old houses.
Mais quand l’adjectif est en fonction de prédicat, on le place après le nom auquel il se réfère.
This writer is famous.
--- POUR APPROFONDIR LE SUJET ---
There are two main kinds of adjectives: attributive ones normally come right before the noun they qualify, while predicative adjectives come after to be or similar verbs such as become and seem. Most adjectives can serve either purpose: we can speak of a 'happy family' and say 'the family appeared happy.' But some work only one way. Take the sentence 'Clergymen are answerable to a higher authority.' Answerable is exclusively a predicative; you could not refer to an 'answerable clergyman.' And higher is strictly attributive; you wouldn't normally say, 'The authority is higher.'