Conjugating 이다 to the past tense is simple, and is done by connecting ~이 to ~었~. When the last syllable in a word ends in a vowel, ~이 + ~었 can combine to make ~였.
Informal Low Respect
Add ~이었어 to all words. If the word ends in a vowel, ~이었어 can contract to ~였어.
나는 바쁜 선생님이었어 = I was a busy teacher
나는 학생이었어 = I was a student
나는 선생님이었어 = I was a teacher
나는 나쁜 애기였어 = I was a bad baby
나는 나쁜 의사였어 = I was a bad doctor
Informal High Respect
This conjugation is the same as above (Informal Low Respect), except for that “~요” is added to the end of ~이었 or ~였. That is, you should add ~이었어요 to all words. If the word ends in a vowel, ~이었어요 can contract to ~였어요.
그것은 큰 비밀이었어요 = That was a big secret
저는 선생님이었어요 = I was a teacher
저는 의사였어요 = I was a doctor
저는 나쁜 애기였어요 = I was a bad baby