What's the difference?
Yo estaba estudiando Yo estuve estudiando
A simple way to explain it would be with examples.
Estuve estudiando = I was studying (but then I did something else). Estaba esudiando = I was studying (when the phone rang).
Does this make sense. One is complete (preterite) and the other incomplete (imperfect). As is often the case, the context determines which is used or what is implied.
Hope this helps.
"Estuve" is the Simple Past (Preterit) tense of estar. I suppose essentially it translates as "I was".
Estaba is the Past Imperfect (Imperfecto) tense of ester. I suppose essentially it translates as "I was sort of continuously, or I used to".
To look at the differences between the Preterit and the Imperfecto click on this link and if necessary on the links it refers you to. Click here ----> link text