Hola A

I have now tagged two lessons that clarify the use of one tense or another, the Indefinido and the Imperfect. I hope that helps. 

Here are the two lessons:

Using El Pretérito Imperfecto for descriptions in the past

Using El Pretérito Indefinido for completed actions in the past

Those are general lessons for A2 in terms of using a tense or another.

These will clarify whether to use estaba vs estuve,  or fui vs era. 

We do have more lessons that explain when to use estar or ser, independently of the tense. For this, you can click here and scroll down to see all the lessons we have on these two verbs:

Ser or Estar 

I hope it helps.

Saludos