Aorist or imperfect
Both are past tenses, but they look at events differently. The aorist is what happened and is over: Dje lexova një libër — "Yesterday I read a book". The imperfect is what was going on or used to happen: Lexoja çdo mbrëmje — "I used to read every evening".
Event and background
In a story the aorist moves the action forward and the imperfect paints the background: Ishte vonë dhe binte shi. Hyra në një kafene dhe porosita një çaj — "It was late and it was raining. I went into a café and ordered a tea". If one action interrupted another, the interrupted one is in the imperfect and what happened is in the aorist: Ndërsa gatuaja, ra telefoni — "While I was cooking, the phone rang".
Once or usually
Once — the aorist: Dje shkova në punë me autobus — "Yesterday I went to work by bus". A habit — the imperfect: Çdo ditë shkoja në punë me biçikletë — "Every day I cycled to work". Clues: dje, një ditë, papritur "suddenly" usually mean the aorist; çdo ditë, gjithmonë, shpesh, zakonisht usually mean the imperfect.
How long it lasted: the aorist
If you say how long an action lasted and it is over, use the aorist: Jetova dy vjet në Tiranë — "I lived in Tirana for two years", Punova gjithë ditën — "I worked all day". Jetoja në Tiranë means "I was living in Tirana at the time": background, with no time limit.
Was and had
Jam and kam are usually in the imperfect when talking about the past: Dje isha në Durrës — "Yesterday I was in Durrës", Kisha shumë punë — "I had a lot of work". The aorist pata is for something that happened: Pata një problem me makinën — "I had a problem with the car".