The -e rule in short
Dutch adjectives (like mooi, groot, slim) sometimes change form. The key rule: with de-words in the indefinite form (een), the adjective gets -e, but with het-words in the indefinite form it does not. In every other case — definite form, plural, possessives and demonstratives — it almost always gets -e.
Adjectives for materials
Materials follow their own rule: most take -en. Think of hout (wood) → houten, glas (glass) → glazen, metaal (metal) → metalen, goud (gold) → gouden, zilver (silver) → zilveren, leer (leather) → leren and katoen (cotton) → katoenen.
Exceptions: no -en
A few materials — usually more modern or borrowed words — don't follow this rule: aluminium, nylon, polyester and kunststof (plastic) stay unchanged.