Most German adjectives can serve a dual purpose in the grammar - one where they are used as adjectives to apply their meaning to nouns, and one where they are used as adjectives and apply their meaning to an action (verb) or another adjective. The best and most frequent example is "gut," which is translated into English as either "good" (adjective) or "well" (adverb). The examples below illustrate different contexts in which "gut" can appear.
|1. Der Film war sehr gut.||The film was very good.|
|2. Ich habe gut geschlafen.||I have slept well.|
|3. Der Aufsatz war gut geschrieben.||The essay was well written.|
Determining whether an adjective can be used as an adverb is not usually too difficult, although if you are in doubt, bilingual dictionaries indicate whether an adjective can be used this way be listing an adverb as a translation. Typically, it comes down to whether it would make sense as an adverb or not. So, for example, "rot" cannot be used as an adverb to modify a verb's action because it doesn't make sense to describe actions as "redly." On the other hand, it can be applied to another adjective, as in "rot gestrichene Wände" (lit. "redly painted walls"). If in doubt, look it up!