Someone wrote in another comment that "de" words are masculine or feminine in German and the "het" words are neuter. So it is der Mann and de man, der Hind and de hond, die Frau and de vrouw, der Kuh and die koe. German diminutives usually end with "...chen" or "...lein" and are das words. "Konijn" is a Dutch diminutive and it is a het word. In German, it is das Pferd and in Dutch, it is het paard.
Any "het" word must have the "het" present in order to use the -e at the end of the noun defining adjective. Any "de" word will always have the -e present. If the adjective follows the verb like in "The boy IS..." then it will drop the e, no matter if the word is "het" or "de." I hope that was simple enough
snel (adj) quick, fast, rapid
So the 'e' is a short vowel sound
If you spell it as snele then the first e would be a long vowel sound ee because it would be an open vowel
[ sne ][ le ] (split into syllables)
So to keep the short vowel sound you need the double l
[ snel ][ le ]
Because snel is not a superlative.
snel - sneller - het snelst
fast - faster - the fastest
Het snelle paard (the fast horse) (doesn't mean that there aren't horses that are faster than him)
Het sneller paard (the faster horse) (again doesn't mean there aren't any faster than him)
Het snelst paard (the fastest horse) (there's only one of him)