- Add -en to form the infinitive
- First, take off the -en (the infinitive marker) to get the stem. If there's a double consonant (e.g.,-kk) left at the end of the word, take away one of them.
- Second, add -t to the stem if the last letter is voiceless, and add -d if the last letter is voiced. If the stem ends in -d, don't add anything.
- Lastly, if the stem ends in -l, double the vowel (e.g., betalen "to pay" => betal-en => betal- => betaal- => betaald).
See more here Dutch Grammar • The past participle
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