passé composé vs. l'imparfait

Question

In the sentence "donc ce n'était pas trop compliqué !"Why was l'imparfait used? I thought lepassé composé would be a best fit for the sentence.

Accepted Answer

Bonjour Luciana,
As Jim mentioned, the sentence describes an opinion/a state of facts about a situation, in which case l'imparfait will be used. 
See link to Kwiziq lesson: https://french.kwiziq.com/revision/grammar/l-imparfait-usage-describing-and-expressing-opinions-in-the-past-imperfect-tense
Plus as mentioned in the lesson (see link above), when the verb être is used in a past context, it usually is in L'Imparfait as it describes something/fact or as it expresses an opinion. 
For example:

c'était bien! 
It was great!
Le test était horrible et compliqué!
The test was horrible and complicated!

I hope it is helpful. 
Bonne journée!

Answer

As I see it the text is describing a "state" of complication. Être is appropriate but the situation historically and enduring, therefore était. The point is the "enduring historic situation" rather than "completed."

Hope this helps?