1. The Imperfect Progressive tense (el Imperfecto Progresivo)
To tell what is happening now, you use the Present Progressive tense:
The Present tense of ESTAR + Present Participle
I am studying. You are working
Estoy estudiando. Tú estás trabajando

2. The Imperfect Progressive is used to describe an action as it was taking place in the past. References to such actions usually omit mention of a specific length of time. Like the Present Perfect, you use ESTAR and the Present Participle. Since these actions are referred to as they were taking place (over a period of time), ESTAR is used in the Imperfect.
estaba estábamos
estabas estábais
estaba estaban

3. The laborer was working in the factory.
El obrero estaba trabajando en la fábrica.
The boys were eating the fruits from the orchard.
Los muchachos estaban comiendo las frutas del huerto.

4. Most verbs that have a stem change in the 3rd person in the preterite have the same stem change in the Present Participle:
e>i o>u
Pedir pidiendo dormir durmiendo
Servir sirviendo morir muriendo
Repetir repitiendo
Decir diciendo
Also, as in the 3rd person Preterite, some verbs have a “y” in place of the “i” in the Present Participle:
Caer cayendo construir construyendo
Leer leyendo contribuir contribuyendo
Traer trayendo oír oyendo

5. I was waiting for the bus.
Yo estaba esperando el autobús.
Were you walking through the park?
¿Estabas andando por el parque?
Beto was crossing the street.
Beto estaba cruzando la calle.
We were reading the book.
Estábamos leyendo el libro.
They were sleeping when Ana called.
Ellos estaban durmiendo cuando llamó Ana.
What were you saying when I arrived?
¿Qué estabas diciendo cuando llegué?