Learn Dutch: Je moet zo vroeg mogelijk vertrekken. - You have to leave as early as possible.

In Dutch, the phrase zo [adverb/adjective] mogelijk is a fixed pattern meaning "as [adverb/adjective] as possible". So zo vroeg mogelijk means "as early as possible". You do not use the Dutch word for "as" (als) in this specific construction.

Why is vertrekken at the very end of the sentence?

This is a standard Dutch word order rule. Because the sentence uses the modal verb moet (have to / must) in the second position, the main verb (vertrekken) is pushed to the very end of the sentence in its full dictionary form (the infinitive).

How would I say "as fast as possible" using this same pattern?

You would use the exact same structure: zo snel mogelijk. You can easily swap in other words between zo and mogelijk to make new phrases, like zo goed mogelijk (as good/well as possible).

Could I use jij instead of je at the start of this sentence?

Yes, but it changes the tone slightly. You would use jij if you want to place emphasis on the person (e.g., "You have to leave, not me"). Because there is no special emphasis here, the unstressed form je is the most natural choice.