だった could be an inflection of だ, with these forms:
- Ta-form. It indicates the past tense of the verb.
More reading
The word だ, and its polite form です, is similar to the English word “is”. Here are a few good articles to learn more.
- Tae Kim's Guide to Learning Japanese: Expressing state-of-being, Formal Expressions (である、ではない)
- imabi: Copular Sentences I, Copular Sentences II, Copula Verb Variants
Sentences — 3303 found
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jreibun/3637/2
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ちょうど希望に合うツアーを見つけたが、
- さいしょうけいこうにんずう最少催行人数 が
- ごめい五名 となっている。参加希望者が私たち
- ふたり二人 だけだったら、成立しない。
I found a tour that meets our preferences perfectly, but the minimum number of participants is five. If we two are the only people who want to join the tour, it will be cancelled. — Jreibun