Pasado progresivo

Past Progressive/Continuous - Use
The Past Progressive is used when we talk about something which was happening at a special time in the past. It is also called Past Continuous. Have a look at the following examples:

1) action was in progress at special time in the past
Peter was reading a book yesterday evening.
She was listening to the radio.

2) two actions were happening at the same time (the actions do not influence each other)
Anne was writing a letter while Steve was reading the New York Times.

3) together with the Simple Past
While we were sitting at the breakfast table, the telephone rang.

Note:
Past Progressive: were sitting at the table
Simple Past: the telephone rang.

The action in the Simple Past interrupted the action in the Past Progressive.

4) repeated actions irritating the speaker (with always, constantly, forever)
Andrew was always coming in late. (I don't like it.)

Simple Past:
Andrew always came late. (Here I don't give a comment.)

Past Progressive/Continuous - Signal words
Signal words tell you what tense you have to use. In the Past Progressive we often use a period of time. A typical signal word is while. The Past Progressive is frequently used in sentences together with the Simple Past.

while (In this part of the sentence we usually use Past Progressive.)

While we were watching TV, Andy was surfing the internet.

when (In this part of the sentence we usually use Simple Past. The Past Progressive is used in the other part of the sentence.)

The ladies were talking when the accident happened.

Form of the Past Progressive/Continuous
We use a form of to be (was or were), the infinitive of the verb and the ending –ing.

to be (was, were) + infinitive + -ing

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Affirmative sentences:
I/he/she/it was playing football.
We/you/they were playing football.

NOTE: Use was with I, he, she, it -
and were with all other pronouns.

In affirmative sentences we do not use short forms in the Past Progressive.

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Negative sentences:
I/he/she/it was not playing football.
We/you/they were not playing football.

We use short forms in the Past Progressive in negative sentences:

I/he/she/it wasn't playing football.
We/you/they weren't playing football.

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Questions:
In the Past Progressive we put the auxiliary (was or were) before the subject
(Auxiliary - Subject - Verb - Rest).

Was I/he/she/it playing football?
Were we/you/they playing football?

Past Progressive - Spelling
Be careful with some words when adding -ing to the infinitive.

1) consonant after a short, stressed vowel at the end of the word
Double the consonant.

sit – he was sitting
put - he was putting

If the consonant is not stressed, we do not double it:
benefit - benefiting (Here we stress the first 'e', not the 'i'.)

In British English we double one -l at the end of the word:
travel - travelling

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2) one -e at the end of the word
Leave out the -e.

write – he was writing
take – he was taking

BUT:
double –e: add -ing
see – he was seeing

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3) verbs ending in -ie
Change 'ie' to 'y'.
lie - he was lying

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4) verbs ending in -c
Change 'c' to 'ck'.
picnic - he was picnicking

Fuente: http://www.englisch-hilfen.de/en/exercises/tenses/past_progressive_statements.htm