"In the morning I finally read that short story."
Translation:Утром я наконец прочитал этот рассказ.
I used the imperfective "читала" and was marked wrong. The English could be translated either way, I believe, since it's not clear whether the story was completed, which the perfective suggests. If you are trying to force the use of the perfective, why not change the English to something more like "In the morning I finally read the rest of (or all of, or finished) the story"?
I think you would use почитала then, implying that you had only read some of the story, but not all of it.
Ok, but is читала wrong or awkward? I am thinking especially of those for whom these concepts are genuinely new. One wants to be as clear as possible with new learners when introducing foreign concepts. That's all I'm suggesting.
I would say it sounds awkward because of the "finally" + past tense which implies that the action has been completed (at least for now). Sorry I don't know how to best explain this, but because there's a "finally" there, I don't think читала is the appropriate verb to use.
A cheap trick I use is to substitute the past + gerund to see if it makes sense in English. In this case, "I was finally reading that short story." doesn't make a lot of sense, so I would go with perfective case.