"That's what I thought"
My friend has two brothers and one sister. I thought her sister was the youngest, but she sounded like the younger brother was the youngest. So I said "I thought your sister was the youngest one." Then she said, "Me, Brother 1, Brother 2, and Sister." Year, that's what I thought.
The polite form (or full expression) for だと思った is 「そうだと思いました（そうだとおもいました）」. That sounds more polite.
Autumn of Japan
6 years ago