Learn Dutch: Pas als jij de veiligheidsriem vastmaakt, vertrekt het vliegtuig. - Only when you fasten the seatbelt, does the plane depart.

Because the entire restrictive conditional clause (Pas als jij de veiligheidsriem vastmaakt) takes up the first position in the overall sentence. In Dutch, whenever a sentence starts with anything other than the subject, the conjugated verb must come second, followed by the subject. This rule forces the inversion in the main clause.

Could I use alleen wanneer instead of pas als in this sentence?

Yes, alleen wanneer is grammatically correct and means essentially the same thing here. However, pas als (meaning 'not until' or 'only once') strongly implies a timeline or a delayed outcome, whereas alleen wanneer focuses purely on the strict condition itself.

Since vastmaken is a separable verb, why is vastmaakt written as a single word here?

In a standard main clause, the verb would separate (for example, Jij maakt de riem vast). However, this is a subordinate clause introduced by the conjunction als. Subordinating conjunctions push all verbs to the end of the clause, causing the separable prefix (vast-) and the conjugated verb (-maakt) to snap back together into a single word.

Is there a specific reason to use jij instead of je in this sentence?