Learn Dutch: Het dier beweegt geluidloos door het donkere bos. - The animal moves soundlessly through the dark forest.

The suffix -loos means without or -less. By attaching it to the noun geluid (sound), you create the adjective geluidloos (soundless or without sound).

The lesson is about adjectives, but geluidloos translates to soundlessly. Why doesn't it have an adverbial suffix?

In Dutch, the base adjective form is almost always used directly as an adverb without adding an extra suffix like -ly in English. Because it modifies the verb beweegt here rather than a noun, geluidloos acts as an adverb and translates as soundlessly.

Why does donkere have an -e added to it, but geluidloos does not?

Geluidloos is functioning as an adverb here to describe how the animal moves, and adverbs never take an -e ending. Donkere, on the other hand, is an adjective directly modifying the noun bos. Because bos is a neuter noun and is preceded by the definite article het, the adjective donker requires the standard -e ending.

Could I just say that the animal moves stil through the forest instead?