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を開けると

when ... opens

→ を
phrase Rare #26,246
when ... opens Used with an object marked by を to describe what happens when or as soon as someone opens it. The subject is often understood from context.
JA: 窓を開けると、涼しい風が入ってきた。
EN: When I opened the window, a cool breeze came in.

トムが扉を開けると、犬が走り出てきた。

When Tom opened the door, a dog came running out.

床のフタを開けると下水に出られる

If you open the floor hatch, you can get out into the sewer.

— REC / レック (2007)

私がかごを開けるとすぐに、カナリアは飛び出した。

As soon as I opened the cage, the canary flew out.

口を開けるとヘンよ！

You look funny when you open your mouth!

— かぐや姫の物語 (2013)