I Sara, Sometimes translating doesn't help to understand a language. In Spanish we have 5 basic past tenses. The best way to approach them is learning their use. For example: Pasado imperfecto is used for habits or things that happened in a period of time in the past. Estar+gerundio can be used with all the tenses. It gives them more proximity. So a change of the point view. I hope it helps you, Saludos!

For the use of describing momentary situations in the apst, in other words, something that is happening in a particular moment of the past, you can use both Imperfecto o Imperfecto + gerundio with the same meaning. Hablaba con ella cuando me llamaste. Estaba hablando con ella cuando me llamaste. I guess in English, since you don't have a tense like the imperfect, for this use you would translate it using past continuous.

I really liked your question. I have never reflected on this.