Learn French: Ils ont exploré de nouveaux lieux.

In French, most nouns and adjectives that end in -eau or -eu take an -x instead of an -s in the plural. This is why the masculine singular nouveau becomes nouveaux and lieu becomes lieux.

What are the singular forms of nouveaux and lieux?

The singular of nouveaux is nouveau (new, masculine), and the singular of lieux is lieu (place). So, "a new place" would be un nouveau lieu.

Why is it "de nouveaux lieux" instead of "des nouveaux lieux"?

This is a tricky rule! The plural word for "some", des, changes to de when it comes directly before a plural adjective. Since the adjective nouveaux comes before the noun here, des becomes de.

Doesn't the adjective usually go after the noun in French? Why is nouveaux before lieux?

Usually, yes! But certain short, common adjectives go before the noun. A helpful trick is to remember the acronym BANGS (Beauty, Age, Number, Goodness, Size). Nouveau describes age or newness, so it goes in front of the noun.