Learn German: Tom riskiert das Picknick, zumal er schon alles vorbereitet hat. - Tom is risking the picnic, especially since he has already prepared everything.

zumal translates to "especially since" or "particularly because". It is a subordinating conjunction, which means it introduces a dependent clause and forces the conjugated verb (hat) to the very end of the sentence.

How would we rewrite this sentence to use the lesson's focus, es sei denn (unless)?

Because zumal gives a reason rather than a condition, you would have to change the sentence's logic to use es sei denn. For example: Tom riskiert das Picknick nicht, es sei denn, er hat schon alles vorbereitet (Tom is not risking the picnic unless he has already prepared everything). Notice that es sei denn is usually followed by normal main clause word order, unlike the "verb-at-the-end" rule used for zumal.

Why does vorbereitet come right before hat at the end of the clause?

Because the conjunction zumal pushes the conjugated helping verb (hat) to the very end, it ends up after the past participle (vorbereitet). In standard main clauses, the conjugated verb is in position two and the participle is at the end, but in subordinate clauses, they stack at the end with the conjugated verb coming last.

Why is it alles instead of alle here?