Learn French: Il n'a jamais vu les nouveaux châteaux. - He has never seen the new castles.

In French, most nouns and adjectives ending in -eau or -eu take an -x to form their plural, rather than the usual -s. The singular masculine forms here are nouveau and château.

Why does nouveaux come before the noun châteaux?

Most French adjectives go after the noun. However, short and common adjectives that describe Beauty, Age, Goodness, or Size (often remembered as the BAGS rule) go before the noun. Nouveau describes Age, so it is placed before châteaux.

Why is jamais placed between a and vu?

In the passé composé (past tense), negative expressions like ne ... jamais (never) wrap around the helper verb (a), not the main action verb (vu). So instead of putting jamais at the end, it goes right in the middle: il n'a jamais vu.

Do you pronounce the -x at the end of nouveaux and châteaux?

No, the -x at the end of these words is completely silent. The singular nouveau and plural nouveaux sound exactly the same when spoken, just like château and châteaux. You know they are plural in spoken French because you hear the plural article les before them.