"Last night the moon was very bright."
Translation:어젯밤에 달이 아주 밝았다.
I agree, both "밝았어요" & "밝았다" should be interpreted the same way, and accepted
저는 그렇게 아닐 것 같은데요. 밝았다 would be used more in written language or when you talk to yourself as an exclamation, while 밝았어요 sounds rather relaxed and more like talking to a friend who is older than you.