だと思ってた

thought it was

→ だ
phrase Uncommon #16,855
thought it was Informal contraction of だと思っていた. Used to describe a past belief or assumption, often one that turned out to be wrong. The subject can be understood from context.
JA: 今日は休みだと思ってた。
EN: I thought today was a day off.

カヨ竜宮城だと思ってた

(Kayo) Did you think it was Ryugu Castle?

— 崖の上のポニョ (2008)

あいつは千草だと思ってたけどな

I thought that guy was Chigusa, though.

— バトル・ロワイアル (2000)

友達だと思ってたのに

I thought we were friends.

あなたの仕事だと思ってた

I thought it was your job.

お前の仲間だあんたがこれ以上だと思ってたあんたも白人と同じだ

You're one of them. I thought you were better than this, but you're just like the white man.

— 罪人たち (2025)

それはトムだと思ってた

I thought that was Tom's.

ずっと犬は全部オス猫は全部メスだと思ってた

A long time ago, I thought all dogs were male and all cats were female.

みんな私のパーティーに来るつもりなのだと思ってた

I thought everyone was planning to come to my party.

リュウまあまあだと思ってた

I thought Ryu was pretty decent, but...

— イップ・マン 序章 (2008)

トム医者だと思ってた

I thought Tom was a doctor.

友達だと思ってたのに

I thought we were friends!

彼女は私が医者だと思ってたのよ

She thought I was a doctor.

そんなことだと思ってた

I had a feeling it was something like that.