Questions & Answers about Вчера Анна долго читала журнал.
Why is the imperfective verb читала used here instead of the perfective form?
The key word here is долго (for a long time). Whenever you want to emphasize that an action was a continuous process or took a long time to happen, you use the imperfective aspect. If Anna had read the magazine from start to finish and we wanted to focus on a single completed result, we would use the perfective verb прочитала.
Does читала mean 'read' or 'was reading'?
It can mean both depending on the context! Because it is an imperfective verb paired with долго, 'was reading' is a very natural English translation to show it was an ongoing process. English forces us to choose between 'read' and 'was reading', but Russian uses the same imperfective past tense form for both.
Why does the verb читала end in -ла?
I know direct objects take the accusative case. Why didn't the ending of журнал change?
You are right that журнал (magazine) is the direct object here and is in the accusative case. The trick is that for inanimate masculine nouns (objects, things that aren't alive), the accusative form is exactly the same as the dictionary form. It only gets a new ending if it is a living person or animal.