"De snelle schildpadden"
Translation:The fast turtles
Don't be confused about the extra L added alongside the E. 'Snelle' is used before nouns as a direct adjective. e.g. Een snelle vis - A quick fish. If I had said instead 'A fish is fast' it would be 'Een vis is snel'. So, the 'le' is added to the end when the adjective comes directly preceding a noun. Most of the time, adjectives directly before nouns get an 'e' ending on its own, but Snelle is an exception. This rule does not apply to adjectives preceding singular 'het' nouns.
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