Adjectives as Predicates and Attributes
Adverbs
The dictionary form (unmarked form) of a German adjective can also be used as an adverb.
Das Buch ist gut. (predicate)
Das Buch ist gut geschrieben. (adverb)
Mein Vater ist streng. (predicate)
Das ist streng verboten! (adverb)
In English, most adjectives have related adverbs that are marked by the ending -ly:
This is a strong feeling. (attribute)
I feel strongly about it. (adverb)
Children may use the "German system" sometimes, in a more or less slangy way: I did good! Just remember that the Germans use it all the time, and it's correct.
Predicate Adjectives
As in English, German adjectives are used
in the predicates after a copulative verb (like 'sein' or 'werden'): das Buch ist gut
to modify nouns (as an "attribute" preceding the noun) ein gutes Buch
In German, predicate adjectives have only one form, the same form as the adverb:
Das Buch ist gut.
Die Bücher sind gut.
Der Blumengarten ist schön.
Die Frau ist schön.