Dutch has two grammatical genders: common (de-words) and neuter (het-words). Roughly 75 percent of Dutch nouns are de-words and 25 percent are het-words. Unlike German, Dutch has merged masculine and feminine into one common gender, so de covers both. The gender of a noun affects the article (de vs het), the adjective endings in certain positions and the relative pronoun (die vs dat). There is no reliable method to determine gender from sound alone — it must largely be memorised alongside the noun.
Some patterns help. All diminutives are het-words: het huisje, het kopje. All infinitives used as nouns are het-words: het lopen, het eten. Words ending in -ing, -heid, -nis and abstract -st are typically de-words: de vergadering, de schoonheid, de gevangenis. Words ending in -ment, -isme and -eum are typically het-words: het moment, het realisme, het museum. These patterns cover a useful subset of the vocabulary.
The practical advice: always learn nouns with their article — not just huis but het huis, not just dag but de dag. Flashcard apps that include articles from the start build the right habit. When uncertain, guess de — you will be right three times out of four. With het-words specifically, the impact of getting gender wrong is visible in adjective inflection (een groot huis, not een grote huis) and relative pronouns, so het-words deserve extra attention and deliberate practice.