This is the 74th most frequent Japanese word.
思う
"To think" or "to feel."
In this sentence, '思う' indicates a level of uncertainty or conjecture based on personal belief.
彼が本当にそう言ったかどうかは分からないけど、そうだったと思う。
I don't know for sure if he really said so, but I think he did.
Here, '思う' is used to express personal opinion about a past event.
私は昨日の映画は面白かったと思う。
I think the movie we watched yesterday was interesting.
Here, '思う' conveys the speaker's intention or decision about a future action.