Double vowels are long vowels. Even though they consist of two vowels, they are pronounced as one.
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aa | Pronounce like English 'a' but open your mouth (a lot!) wider: [a]
staan (to stand)
waar (true, where)
kaart (card, map)
ee | Pronounce like English 'ai' in claim: [e]
zee (sea)
kleed (carpet)
been (leg)
geel (yellow)
But: pronounce like English 'ea' in beard when followed by the letter r:
meer (more)
zeer (very)
verkeerd (wrong)
heersen (to reign)
oo | Pronounce like English 'oa' in boat: [oo]
rood (red)
hoofd (head)
groot (big)
troosten (to comfort)
But: when it precedes the letter r, pronounce like the English _oo_ in door: [ɔː]
woord (word)
vermoorden (to kill)
noord (north)
Noors (Norwegian)
uu | Pronounce like French 'u' in 'tu' or German 'ü' in Düsseldorf: [y]
vuur (fire)
uur (hour)
muur (wall)
duur (expensive)
debuut (debut)
staan, waar, kaart zee, kleed, been, geel meer, zeer, verkeerd, heersen rood, hoofd, groot, troosten woord, vermoorden, noord, Noors vuur, uur, muur, duur, debuut