What is the imperfect tense?
An imperfect tense is used to indicate that the action may not be completed.
If you say: "I was reading a book.", we do not actually now if you finnished the book or not.
We cannot decide if the action is finnished or not, hence this sentence is imperfect.
The perfect equivalent would be I have read a book."
This sentence informs us about completing the action, hence it is a prefect one.