Learn French: Leurs choix sont toujours surprenants. - Their choices are always surprising.

When a present participle ending in -ant is used purely as an adjective to describe a noun, it follows the normal adjective rules and agrees in gender and number. Since it describes the masculine plural noun choix, we add an s. If it were acting as an actual verb rather than an adjective, it would stay as surprenant.

How do we know choix is plural in this sentence?

Nouns that end in -s, -x, or -z in their singular form do not change when they become plural. Un choix (a choice) becomes des choix (choices). We know it is plural here because the words around it tell us: the plural possessive leurs and the plural verb sont.

I often forget when to put an s on leur. Why does it have one here?

As a possessive adjective, leur means "their" and it must agree with the thing being owned, not the people who own it. Since the thing being owned is plural (choix), you use the plural form leurs. If they had made only one choice, it would be leur choix.

Why is toujours placed after sont instead of before it?