Most plural nouns are formed by adding either -en or -s.
Remember that the definite article is always de before plural nouns.

1. -en (the n is pronounced softly) is added to most
nouns, with a few spelling changes

boek - boeken book(s)
jas - jassen coat(s)
haar - haren hair(s)
huis - huizen house(s)

Spelling changes: Words with long vowels (aa, ee, oo, and uu)
drop the one vowel when another syllable is added. Words with the short vowels
(a, e, i, o and u) double the following consonant to keep the vowels short.
The letters f and s occur at the end of words or before consonants, while the
letters v and z occur in the middle of words before vowels. (These spelling
rules are also used for conjugating verbs, so it's best to memorize them as
soon as possible.)

2. -s is added to nouns ending in the unstressed syllables
-el, -em, -en, and -er (and -aar(d), -erd, -ier when referring
to people), foreign words and to most nouns ending in an unstressed vowel

tafel - tafels table(s)
jongen - jongens boy(s)
tante - tantes aunt(s)
bakker - bakkers baker(s)

Nouns ending in the vowels -a, -o, and -u add an apostrophe before
the s: foto's, paraplu's

Irregular forms

3. Some nouns containing a short vowel do not double the following
consonant in the plural before -en. The plural vowel is then pronounced as long.

bad - baden bath(s)
dag - dagen day(s)
spel - spelen game(s) (like the Olympics, smaller games are spellen)
glas - glazen glass(es)
weg - wegen road(s)

4. A few neuter nouns take the ending -eren (or -deren if the noun
ends in -n)

blad - bladeren leaf(leaves)
kind - kinderen child(ren)
ei - eieren egg(s)
been - beenderen bone(s) [Note: been - benen leg(s)]
lied - liederen song(s)
volk - volkeren nation(s), people

5. Nouns ending in -heid have a plural in -heden.

mogelijkheid - mogelijkheden possibility(ies)

6. Some other common irregular plurals are:

stad - steden town(s)
schip - schepen ship(s)
lid - leden member(s)
koe - koeien cow(s)

This article was used with permission from:
Indo-European Languages