Learn Dutch: Zij rent het snelst en breekt het record. - She runs the fastest and breaks the record.

Snelst is acting as an adverb here because it describes how she runs (the verb rent), rather than describing a noun. When a superlative modifies a verb in Dutch, you always use the fixed phrase het + superlative. You don't add an -e to the end like you would with an adjective.

How would the superlative change if I wanted to say "She is the fastest"?

If you use it as an adjective describing the pronoun "she," you would say: Zij is de snelste. You use de and add the -e ending because it refers back to a person. But in our current sentence, het snelst is locked in because it modifies the action of running.

Is the word record pronounced like the English word?

Yes! Dutch borrowed the word record from English to talk about sports and achievements. It is pronounced almost exactly like the English noun, though usually with a slightly rolled Dutch 'r'. Notice that it is a neuter noun, which is why it uses the article het.

Why doesn't the second part of the sentence say en zij breekt?