het is both the and it, de is only the' xMerrie says you can technically use de, but its a gerund which inherently means its a het word. a gerund is a verb used as noun, snel or fast is verb, so in the sentence it is used as noun snelste is gerund making it het word not de word, but xMerrie is mod so their word is authority while i am just learning. there are differnt ways to tell if it is het word for example all sports and metals, and gerunds, diminutives, all languages ( het Engels, het Frans) All points of the compass (het noorden, het oosten),all names of continents, countries, cities, villages & all words in only 2 syllables starting with one of these prefixes be ge ont ver, het begin, het beroep het gebak, het gebit het ontbijt het verbod, ect. and finally all words ending in - en, isme, ment, um. But then there are a few others just because of the way it feels or for no objective justification in particularly described, but i don't think there are that many like that to make it confusing once you see what is truly het with the basic universal het rules.
In English you can say either of the following:
1a. He is fastest.
2a. He is the fastest.
Dutch also has two possibilities. The sentences above translate as:
1b. Hij is het snelst.
2b. Hij is de snelste.
In Dutch, the word "het" can mean "it", or it can be the neuter definite article meaning "the". But "het" also has a third function: It is a marker of the superlative, regardless of gender.
Thus, in 1b "het" does not refer to the subject "hij" and is not "neuter"; it is simply a marker for the superlative. That is why the best translation into English of 1b is 1a -- because 1a says simply "fastest" and not "the fastest".
In the English 2a, there is a real definite article and the sentence is the equivalent of "He is the fastest one. In other words, the one is understood but omitted in 2a.
You can understand the Dutch 2b as also using a real definite article plus an adjective, with the one omitted. But now, because the article is real, it must agree in gender with its referent. And furthermore, the adjective after it must take an -e ending, as do all Dutch adjectives preceded by a definite article. So, for example:
3a. That horse is the fastest.
3b. Dat paard is het snelste.
In other words, with a neuter noun you can say either "Dat paard is het snelst", which follows the model of 1b; or you can say "Dat paard is het snelste", which follows the model of 2b.