1. IMPERFECT PROGRESSIVE AND PRETERITE.
As the figure indicates, the present progressive is used to tell us what was happening. It is formed by estar + present participle (ndo ending). On the other hand, the preterite is used to talk about actions completed at a specific moment in the past or a specific occurrence that took place. Therefore:
2. Maricarmen estaba leyendo el periódica cuando vió el accidente.
Estaba is the conjugation of the verb estar for the third person singular in the imperfect. Leyendo is the present participle of the verb leer and vió is the conjugation of the verb ver for the third person singular in the preterite.
3. Yo estaba pidiendo una silla de ruedas cuando me caí
Estaba is the conjugation of the verb estar for the first person singular in the imperfect as well. Pidiendo is the present participle of the verb pedir and caí is the conjugation of the verb caer for the first person singular in the preterite. Also, me is the reflexive pronoun that also matches the first person singular.
4. Tú estabas vistiéndote cuando sentiste dolor
Estabas is the conjugation of the verb estar for the second person singular (informal form) in the imperfect. Vistiéndote comes from vestirse and includes the reflexive pronoun te.
2. REWRITE SENTENCES.
We can attach after the conjugated present participle. You just need add an accent over the a or e of the participle ending and put the object pronoun before the conjugated verb estar. So:
1. Tía Luisa estaba trayéndome un sandwich
The action here is being performed for me
2. Cristina y Julia estaban pidiéndonos ayuda
The action here is being performed for us
3. Tú estabas duchándote
The action here is being performed for you
4. Nosotros estábamos levantándonos
The action here is being performed for us