The Spanish imperfect progressive vs the Spanish preterite progressive (was +ing)

In Spanish there are two forms that can be used where in English you would use the past progressive tense "was/were +ing".

Which form you need depends on the timeframe of the ongoing action in the past. 

1. El Imperfecto progresivo

For example:

We use the Imperfecto progresivo in these cases:

• When the ongoing action is simply describing "what was happening" without any emphasis or implication about start or end time (see the first three examples above).
• When it is combined with another action that interrupts the ongoing action at that time. The interrupting action is generally in the simple past (see the last example where the ongoing action "She was washing her hair" was interrupted by another sudden action "he arrived".

2. El Indefinido Progresivo

For example:

We use the Indefinido progresivo in this case:

• When the ongoing past action is inside a completed timeframe in the past. See how all examples above contain a time phrase: "toda la noche", "de 6 a 7 de la tarde", "el sábado pasado".

Careful:

Sometimes the timeframe isn't specific about having a beginning and an end so you may find this type of sentences using El Indefinido Progresivo [estuve+gerund]

We don't know when the "eating" started, but the end is expressed with "hasta que acabamos con  toda la comida". This may sound as if that is an interrupting action and may lead you to wrongly think you need to use El Imperfecto Progresivo [estaba+present participle].

See also Using the imperfect tense in Spanish to "describe" a situation in the past.

Remember that some present participles/gerunds are irregular. See:

• List of Spanish irregular gerunds/present participles of the form -yendo
• List of Spanish irregular gerunds/present participles of the form -iendo

To remember how to form the preterite and the imperfect progressive see:

Forming the progressive imperfect with estar + present participle (El Imperfecto progresivo/continuo)

Forming the progressive preterite tense in Spanish with estar + present participle (El Indefinido Progresivo)