Study materials
Picture the power going out one evening, and the next day everyone asking each other: "What were you doing when it happened?"
Answering "I cooked dinner" will not do here: the question is not about the result, it is about what was already under way at that minute. That is what the Past Continuous is for.
We were watching a film when the lights went out.
In progress, not finished
The form is built from two parts you already know: was or were plus the verb with ing. Was goes with I, he, she, it; were goes with you, we, they.
At eight I was cooking dinner.
They were waiting outside.
The difference from the Past Simple shows up in a pair of sentences. The first says the job is done. The second puts you inside the moment while it was still going on.
I cooked dinner.
I was cooking dinner.
A long action and the thing that cut it short
Most often the Past Continuous works in a pair with the Past Simple. The long action runs, the short event breaks into it.
She was sleeping when the phone rang.
I was crossing the street when someone called my name.
Two words keep the roles straight. After when comes the short event; after while comes the long action.
The doorbell rang when I was cooking.
While I was cooking, the doorbell rang.
Setting a scene, and two actions at once
The same tense paints the setting everything happens in. It is the favourite opening move of any story.
The sun was shining and people were walking in the park.
And if two long actions ran side by side, both stay in the Continuous.
I was reading while my sister was studying.
Negatives and questions without did
The rule from was and were applies here too: the helper is already in the sentence, so did is not wanted. In a negative the not attaches to was or were, and the ing form stays where it is.
They weren't sleeping.
Were you waiting long?
What were you doing at ten?
The mistakes everyone makes
The first is calling in did where was or were is already there.
What did you doing? → What were you doing?
The second is losing the ing after a negative.
They weren't sleep. → They weren't sleeping.
The third is putting a state verb in the Continuous.
I was knowing the answer. → I knew the answer.
All three are caught by one question. Was the action already under way at the moment you mean? If so, it takes was or were plus ing, and nothing else.
Try it right now
Think of what you were doing at seven yesterday evening and say it in English.
Then add a short event that could have interrupted you: a call, a message, someone at the door. That is a whole story in two tenses.