The core rule of Dutch adjective inflection: attributive adjectives placed before a noun take an -e ending in most contexts. The only exception is an adjective before a singular neuter (het-word) noun that has an indefinite article (een) or no article. Compare: een mooie dag (de-word, with -e), de mooie dag (with -e), een mooi huis (het-word plus een, no -e), het mooie huis (with -e). This exception trips up learners persistently but is consistent once internalised.
Predicate adjectives — those that appear after the verb such as De dag is mooi and Het huis is mooi — never take -e. This is a completely clean rule with no exceptions. So inflection only affects attributive position. Adjectives after possessives and demonstratives always take -e: mijn mooie tuin, dit mooie huis, die grote kamer. After geen, the adjective takes -e: geen mooie dag, but geen mooi huis behaves like een for het-words.
Spelling changes apply when adding -e: if the adjective stem has a double vowel in a closed syllable, one vowel is dropped: groot becomes grote. If the stem ends in a single consonant after a short vowel, the consonant is doubled: nat becomes natte. These spelling rules mirror those for plural formation and verb conjugation — the same underlying phonological principle governs all three areas of Dutch morphology.