het onderzoek — research
de onderzoeken
Most Dutch nouns add -en (with a spelling tweak).
Every Dutch plural takes de. Check the singular article →
The plural (meervoud) of onderzoek is "de onderzoeken". Most Dutch nouns add -en (with a spelling tweak).
Like every Dutch plural, de onderzoeken takes de. Dutch plurals are always de-words, regardless of the singular article.
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