het onderwerp
het-wordtopic, subject
/ɦɛt ˈɔndərˌʋɛrp/
de onderwerpen
This one you just learn — no rule points to het here.
Wat is het onderwerp van dit boek?
What is the topic of this book?
The de/het checker covers one word. The full platform covers vocabulary, grammar and all five DUO sections.