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이 아니었다

was not

→ 이
phrase Rare #22,921
was not Follows a noun marked with 이, the subject particle; the subject particle may be omitted after nouns ending in a vowel.
KO: 그건 꿈이 아니었다.
EN: That was not a dream.

그녀를 기다리는 건 나뿐이 아니었다

I wasn't the only one waiting for her

— 물랑루즈 (2001)

그건 사실이 아니었다.

That wasn't true.

회의 날짜가 그렇게 정해진 건 우연이 아니었다.

It was no coincidence that the meeting date was set that way.