Si usa il past continuous per parlare di un'azione, o situazione, che era in corso in un momento particolare del passato.
At 7 I was playng football with my friends
Si usa il past Simple (walked) per descrivere un'azione completamente finita. Si usa il past continuous (was writing) per descrivere un'azione in corso nel passato.
When I came home , Mary was coking and Peter was reading.
Si possono usare when o while prima del past continuous.
I saw Julia when she was driving her mum's car
I saw Julia while she was driving her mum's car
Si usa solo when (non while) prima del past Simple
When I saw Julia she was driving her mum's car