Learn French: Je n'ai jamais vu de telles billes sur ce billard. - I have never seen such billiard balls on this pool table.

This is a classic French rule: when a plural adjective comes before a plural noun, the indefinite article des changes to de. Since the adjective telles comes before the plural noun billes, we use de instead of des.

How do we know to use telles here instead of another form like tel or tels?

The word for 'such' here is the indefinite adjective tel. Because it acts as an adjective, it must agree in gender and number with the noun it modifies. Since billes (balls) is feminine and plural, you must use the feminine plural form: telles.

Does the adjective tel always come before the noun it describes?

When tel is used to mean 'such' to express intensity or to point out a certain kind of thing (as in 'such balls' or 'such a situation'), it almost always goes right before the noun.

The English translation says 'billiard balls', but the French just says billes. Why isn't the word billard used to describe the balls themselves?