In the class Adj. 3, which covers comparatives and superlatives: https://www.duolingo.com/skill/dn/Comparative-and-Superlative
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"DE TRAPPEN VAN VERGELIJKING
The "steps of comparison", as they are referred to in Dutch, are used to turn an adjective (good) into a comparative (better) or a superlative (best). These are formed by adding suffixes to the adjective, much like in English. This process is mostly regular, with a few important exceptions. (...)