imperfect indicative conjugation of estar + present participle of verb = imperfect progressive

The present participle (gerund) is formed with the stem of the verb plus -ando (for -ar verbs) or -iendo (for -er and -ir verbs). For example:

hablar ---> hablando (talking)
comer ---> comiendo (eating)
vivir ---> viviendo (living)

Example: the present participle of hablar (to speak) is hablando. The table below shows the conjugation of estar and the proper formation of the imperfect progressive:

The imperfect progressive is used similar to the imperfect.

Él estaba hablando español.
He was speaking Spanish.

Yo estaba leyendo un libro.
I was reading a book.

Nosotros estábamos trabajando.
We were working.

Estaba zigzagueando.
He was zig-zagging.

El niño estaba durmiendo en la tarde.
The boy was sleeping in the evening.