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When I opened the refrigerator door, an apple fell out.
Literal
Refrigerator [of-の] door [object-を] opened [when-たら] apple [subject-が] fell.
~たら functioning as the 'discovery' temporal — 'when I did X, (I found/saw that) Y' — a common ~たら use distinct from straightforward conditionals. 開く here means 'to open' transitively; the door is the object with を. 落ちる ('to fall') is intransitive, so the apple takes が. A mini-narrative in one sentence.