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が到着したとき

when ... arrived

→ が
phrase Rare #42,325
when ... arrived Used to say that something happened when a person or thing arrived. The subject before が can be a person or thing. The exact phrase is often part of a larger sentence, such as 私が到着したとき (when I arrived).
JA: 私が到着したとき、パーティーはすでに終わっていた。
EN: When I arrived, the party had already ended.

救急車が到着したとき開いてた？

Was it open when the ambulance arrived?

— 落下の解剖学 (2023)