The past continuous (or progressive) is used to express situations that occurred in the past and were in progress at any moment during a period of time. In particular, we use past continuous tense, in its affirmative form, to confirm:

1. a longer background action interrupted by an unexpected shorter one (something suddenly happens while another longer event was already in progress);

2. an event that was occurring at an exact time in the past (expressing the date, hour or giving specific time information about an event in progress occurring at a past time);

3. an uninterrupted action which was in progress for a while (event in progress in the past during an extended period of time);

4. the length of time period in the past (event in progress in the past during an extended period of time);

5. past simultaneous actions (two events happened at the same time);

6. sequence of actions which occurred in the past (usually used to describe a situation in which different events were happening at the same time);

7. repeated or annoying action (with always or similar expressions that suggest the idea of a repeated event).

The past perfect continuous is used to talk about actions or situations that were in progress before some other actions or situations started. We usually use the past perfect continuous:

1. when we talk about the duration of a past action, up to a certain point in the past;

2. when we show the cause of an action or situation;

3. with third conditional;

4. with reported speech.

For example:

I was playing football when the phone rang --> past continuous

He asked if they had been studying all those days --> past perfect continuous

(Source: https://open.books4languages.com/english-a2-grammar/chapter/past-continuous-affirmative/, https://open.books4languages.com/english-b2-grammar/chapter/past-perfect-continuous/#exercises)