1.
to be convinced, to be under the impression
To believe something strongly, often without sufficient evidence or despite being incorrect.
彼は正しいと思い込んでいる。
He's convinced he's right.
彼女は自分が悪いと思い込んでいた。
She was convinced that she was at fault.
一度思い込んだら、なかなか考えを変えない。
Once he's convinced of something, he won't easily change his mind.
子供のころ、サンタクロースは本当にいると思い込んでいた。
When I was a child, I was firmly convinced that Santa Claus really existed.
勝手に思い込んでいただけで、事実は違った。
I had just assumed it on my own, but the truth was different.
2.
to assume mistakenly, to jump to conclusions
To believe something to be true based on assumptions rather than facts.
会議は来週だと思い込んでいた。
I was under the impression that the meeting was next week.
思い込みが激しい人だ。
He's a person who jumps to conclusions. / He has strong preconceptions.
[Note: 思い込み is the noun form meaning 'preconception' or 'assumption']
道を間違えたのは、右に曲がると思い込んでいたからだ。
I took a wrong turn because I assumed I should turn right.
彼が犯人だと思い込んでいたが、実は無実だった。
I had assumed he was the culprit, but he was actually innocent.
全員が賛成していると思い込んでいたが、反対の人もいた。
I had assumed everyone was in agreement, but there were people opposed as well.
思い込む is an intransitive verb (自動詞) meaning to be firmly convinced of something.
COMPOUND STRUCTURE:
- 思う (to think) + 込む (to go into deeply) → to think deeply and become fixed in belief
ASPECT (ている):
思い込んでいる is the most common form, indicating a state of being convinced or under an impression.
COMMON PATTERNS:
- ～だと思い込む (be convinced that~)
- 思い込みが激しい (prone to making assumptions)
- 勝手に思い込む (assume on one's own)
RELATED NOUN:
- 思い込み: preconception, assumption, belief
NUANCE:
Often implies that the belief is mistaken or at least unverified. Used when someone has convinced themselves of something that may not be accurate.
[Register: Neutral]