Learn Dutch: Alleen wanneer de piloot bij de gate stopt, mag de deur open. - Only when the pilot stops at the gate, is the door allowed to be open.

The entire conditional clause Alleen wanneer de piloot bij de gate stopt acts as the first position in the main sentence. Because Dutch strictly follows the V2 rule (the main verb must be in the second position), the verb mag comes immediately after the comma, triggering an inversion of the subject (de deur) and the verb.

Could I use pas als instead of alleen wanneer here?

Yes, they are often interchangeable, but there is a subtle difference in focus. Alleen wanneer strictly emphasizes the condition (a rule: "only under this circumstance"). Pas als places a slightly stronger emphasis on time or delay ("not until the moment that"). Both fit perfectly, but alleen wanneer sounds a bit more like a strict aviation protocol.

Why isn't there a verb like zijn or gaan at the very end of the sentence?

In Dutch, when a modal verb like mogen (to be allowed to), moeten (to must), or willen (to want to) is used with a clear direction or state (like open), the main action verb is often dropped. You could say ...mag de deur open gaan (is allowed to open) or ...mag de deur open zijn (is allowed to be open), but dropping the infinitive is much more natural and common.

Why does the verb stopt go at the end of the first clause, despite alleen being at the front?