PERFECTIVE OR IMPREFECTIVE?
Let’s have a look at the following examples:
Tom was doing his homework when I arrived.
Tom did his homework and then he went to the cinema.
In the first sentence, Past Continuous is used, since it describes an unfinished (not completed) activity. The sentence does not determine whether Tom completed the task or not – it says that at a given moment in the past (when I arrived) Tom was just studying.
The second sentence, on the other hand, illustrates the completed, finished task – Tom did his homework, and only then, he went to the cinema. The completed past actions are expressed with the Simple Past tense.
DESCRIPTION OF THE EVENT OR ITS BACKGROUND?
Here we have another two sentences:
I was walking down the street admiring the beautiful Christmas decorations when…
l walked down the street to get to my favourite restaurant.
The first sentence is an introduction to the relation from an event or a description of the background, hence the use of the Past Continuous.
The second sentence describes the event itself in this case – going to a restaurant – which is why we used Simple Past here.
If the sentence where we used Past Continuous refers to the same person performing several actions, there is no need to repeat the verb was/were each time. Thus we will say: l was walking… window-shopping and admiring…
If the remaining actions are performed by another person, it is necessary to use the verb was/were repeatedly and add the appropriate pronoun…
I was cleaning the car and singing.
I was cleaning my car and she was watching.
FEELINGS OR ACTIONS?
Similarly to Present Continuous, we do not use Past Continuous with verbs indicating feelings, state of knowledge, assumptions, etc.
I hated maths at school.
I didn’t remember to feed the dog.
I could hear dogs barking.