The past tense of the plain copula 'だ'. Used to state past conditions or facts (e.g., 'was', 'were').
昨日は雨だったから、試合は中止になった。
It was raining yesterday, so the game was canceled.
彼の話は本当だったと後でわかった。
It turned out later that his story was true.
大学時代は忙しかったが、充実していたと感じていた。
I was busy during university, but I felt it was fulfilling.