Where English uses the -ing gerund as a noun — Swimming is healthy, I enjoy reading — Dutch uses the bare infinitive: Zwemmen is gezond. Ik geniet van lezen. The Dutch infinitive-as-noun is always a het-word: het zwemmen, het lezen, het schrijven. It can take the definite article: Het schrijven van brieven is bijna verdwenen. And it can appear as the object of a preposition: Ze houdt van wandelen. Hij begint met werken.
This differs from the present participle (werkend, zwemmend) which functions as an attributive adjective, not a noun: een zwemmend kind (a swimming child), not the act of swimming. The confusion between -ing as gerund and -ing as participle is an English phenomenon — Dutch keeps the two functions clearly separated. Verbal nouns use the infinitive; attributive verbal modifiers use the present participle added to the word.
Some infinitives have become fully lexicalised nouns with their own meanings: het leven (life as well as to live), het eten (food as well as to eat), het wezen (being or creature). The productivity of the infinitive-as-noun pattern means you can form temporary nominal uses of almost any Dutch verb: het fietsen door de regen (cycling through the rain) is immediately understood even if not in any dictionary.