Maybe you won't believe it, but the adverbs do not have the problem of declination. If it were possible to decline them, Germans would probably do so, but adverbs are invariable and therefore they can't be declined. An adverb is to explain a verb, an adjective or another adverb.

Adverbs that go with verbs     He runs fast.     She writes slowly.     They do it well.

Adverbs that go with adjectives     The car is very slow.     She is too fat.

Adverbs that go with other adverbs     The car can go very fast.     He walks too slowly.