The Dutch plural -en (and its spelling changes)

Most Dutch nouns form their plural by adding -en: de boom → de bomen (the tree → the trees). The ending itself is easy; what trips people up is that adding it often forces a small spelling change so the word still sounds the same. Whatever a noun's singular article was, every plural takes de: het boek → de boeken (the book → the books).

Add -en to the singular noun, then adjust the spelling so the vowel keeps its original length and any final f or s becomes v or z. Three rules cover almost every case.

1. Keep a long vowel long. A long vowel written double (aa, ee, oo, uu) is written single once the syllable opens up: maan → manen (moon → moons), boom → bomen (tree → trees). This is the open-syllable spelling rule at work: ma-nen has an open first syllable, so one a is enough.
2. Keep a short vowel short. A short vowel needs a closed syllable, so you double the following consonant: man → mannen (man → men), kat → katten (cat → cats), pen → pennen (pen → pens).
3. Change f to v and s to z when the letter follows a vowel: brief → brieven (letter → letters), huis → huizen (house → houses), neus → neuzen (nose → noses). Dutch words cannot end in v or z, so the singular hides the voiced sound — the plural brings it back. More on this in why v becomes f and z becomes s.

Treat -en as the default plural: if you do not know a noun's plural, -en is the safer guess. The main group that does something else takes -s instead.

• Most nouns, and nearly all one-syllable words: de dag → de dagen (day → days), de vriend → de vrienden (friend → friends), het huis → de huizen.
• Nouns whose plural ends in -s are a separate set — see the -s and -'s plural and, for how to choose between them, -s or -en?.

The ending -en is the easy part; the spelling adjustment is where errors happen. Writing maanen (keeping both a's) or mannen → manen mixes up vowel length and changes the word: manen is 'moons', mannen is 'men'. Check whether the vowel is long or short before you decide to drop a vowel or double a consonant.

The f/s switch is not universal. When the f or s comes after a consonant, it usually stays: de dans → de dansen (dance → dances), de mens → de mensen (person → people). A handful of vowel + s nouns also keep the s (de eis → de eisen, demand → demands), so treat the change as the strong tendency it is, not an absolute.