Действие в процессе, совершалось в определенный момент или протекало в течение четко ограниченного периода времени в прошлом или происходило в момент совершения другого действия в прошлом.

was / were + Ving

+ They were working at the office the whole evening yesterday.-Они работали в офисе вчера целый вечер.
? Were they working at the office the whole evening yesterday?-Они работали в офисе вчера весь вечер?
- They were not working at the office the whole evening yesterday.-Они не работали вчера в офисе весь вечер.

Слова, которые могут подчеркнуть употребление Past Continuous: whole, still, at … o’clock yesterday, from … till… yesterday, while

Сравните употребление Present Continuous (действие протекает в момент речи) и Past Continuous (действие протекает в определенный момент в прошлом)

Обратите внимание на следующие предложения:

We played chess in the evening. (вечером) – Past Simple
We were playing chess the whole evening (весь вечер) – Past Continuous.

When mother came home, I was reading.
Came – однократное действие (пришла) Past Simple
Was reading – действие в процессе (читал) Past Continuous

When mother was reading, I came home
Was reading – действие в процессе (читал) Past Continuous
Came – однократное действие (пришел) Past Simple

When I was cleaning the room, he was washing the window.
Was cleaning, was washing – два одновременно протекающих действия (убирал, мыл) Past Continuous

Father was reading at 7 o’clock yesterday. Действие в процессе – Past Continuous
Father came home at 7 o’clock yesterday. Однократное действие – Past simple