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