1. Is used to describe something that was taking place over a period of time in the past.
2. Is formed with the imperfect of estar (estaba, estabas, estaba, estábamos, estabáis, estaban) + the present participle (-ar=ando and -er/ir=iendo)
Estaba esquiando cuando me caí y me torcí la rodilla.
3. The present and imperfect progressive use the same present participles.
4. There are irregular present participles for verbs ending in -ir or verbs with doble vowels.
decir=diciendo
pedir=pidiendo
repetir=repitiendo
seguir=siguiendo
servir=sirviendo
vestir=vistiendo
creer=creyendo
leer=leyendo
traer=trayendo