The difference between le passé compose et l'imparfait is clearly explained to a French learner by differentiating between a 'once off' event vs. something habitual or over an extended period of time. This is relatively straight forward.
The issue I have is with the application of 'etre' and 'avoir' when used in the past as stand-alone verbs.
I was at the cinema : J'étais au cinéma quand tu m'as appelé.
In this basic example above, we are using the imparfait form, yet it is a 'once off' event. My being at the cinema wasn't something habitual or over a long period of time...I was there at the moment when you called. Therefore one should use passé compose (j'ai ete au cinema...) but l'imparfait still holds?
I had two slices of cake : J'avais deux tranches de gateau parce que j'adore gateau.
Again, you had two slices at a specific point in time. You CAN say j'ai eu deux tranches de gateau but the imparfait still holds, even for something that was a 'once off'?
Can anyone explain why it is acceptable to use l'imparfait as above?