To say I was or we were, when do we use Imparfait vs Passe Compose?

Question

Example above we have used "Ils ont été" to mean "they were", however I learned elsewhere that to say one was, in the past, we always use imparfait, which in this case would have been "ils étaient". Please help me understand.

Accepted Answer

Even though most of the time "was" in the past will use the imperfect tense instead of passé composé, this is by no means a hard and fast rule.
C'était une soirée très sympa. Le dîner a été super! -- It was a nice evening. The dinner was great!
In this example you'd use the imperfect for the first sentence and the passé composé for the second one. This underlines the fact that the dinner was one event embedded in the entire evening.