An adverb is a word that modifies the meaning of a verb. Most adverbs in German are derived from adjectives, so if you know the adjective you know the adverb. A few important adverbs aren't, however, and these include important examples like nicht, leider, and gerne.
Adverb formation is simpler in German than in most other languages. An adverb is simply the uninflected form of the adjective (or participle). This holds for the positive and comparative forms. The superlative is formed with the preposition am and the ending -en, e.g. am schönsten "most beautifully". Only a limited number of adverbs have a special form ending in -stens, e.g. schnellstens ('as fast as possible'), bestens ('very well').
schnell ("fast, quickly")
groß ("big, substantially")
fließend ("fluent, fluently")
schneller ("faster, more quickly")
fließender ("more fluent, more fluently")
am schönsten ("most beautiful, most beautifully")
The adverb can be used to describe actions, adjectives or other adverbs. Comparative and superlative forms are unusual in the last two situations.
Der Vogel fliegt schnell ("the bird flies fast")
Der Vogel fliegt am schnellsten ("the bird flies the fastest")
Ein schrecklich langsam wachsender Baum ("a terribly slow-growing tree") (literally, "a terribly slowly growing tree")
Ein schneller wachsender Baum ("a faster-growing tree")
In English, adverbs are usually distinguished from adjectives by the ending -ly. As you can see, this isn't the case in German. You can tell adverbs apart from adjectives by the lack of declension. Compare:
ein schrecklich hoher Berg – an awfully high mountain.
ein schrecklicher, hoher Berg - an awful, high mountain.
Adverbs ending in -weise
Unlike English, the German language distinguishes adverbs which qualify verbs or adjectives from those which qualify whole sentences. For the latter case, many German adjectives form a special adverb form ending in -erweise, e.g. glücklicherweise "luckily", traurigerweise "sadly" (from Weise = way, manner).
In the following two example sentences, the adverb lustig "funnily" qualifies the verb, while lustigerweise "funnily" qualifies the whole sentence:
Er hat lustig gesungen. – He sang funnily. (= He sang in a way that was funny.)
Er hat lustigerweise gesungen. – Funnily, he sang. (= It was funny that he sang.)
As in the above example, English usually expresses the difference by placing the adverb which qualifies a sentence, in the beginning. In German, it can be placed in the beginning or elsewhere in the sentence.