Bonjour / Bonsoir.
Ancien is one of those adjectives that changes meaning whether before or after the noun - former when before and ancient when after. However I came across the sentence in a French magazine - "l'un des plus anciens sites du pays", which obviously means one of the most ancient sites in the country. Presumably the inclusion of the 'plus' shifts the 'anciens' to in front of the noun and it retains its meaning of ancient. So "l'un des sites plus anciens du pays" would be wrong ?
