Hey, as a native Spanish speaker I hope I can answer your question: In spanish we have to variants of the "simple past and past continuous" we have the "preterite" and the "preterite imperfect" Therefore; "estuvo aprendiendo..." is using the preterite past "estar:estuvo continuous: aprendiendo, and "estaba aprendiendo" uses the "preterite imperfect past" estar:estaba continuous: aprendiendo. So what is the difference? Well actually, when we speak on a daily basis we don't see any difference but grammatically, there is a difference, when you use " ella estuvo aprendiendo..." you are talking about an action in the past that finished already at an specific time. when you use " ella estaba aprendiendo...." you are talking about an action in the past that is repeated or habitual and has no specific ending. However both in english would be " she was learning to.." there is no such difference in english. Hope I helped.