What is the difference between the past progressive and the past perfect progressive, since both of them express a duration in the past?
Past Perfect and Past Perfect Progressive tenses are both used together with Past Simple and Past Progressive to indicate which of two mentioned past actions happened earlier.
After I had done my homework I started reading a book.
Here the actios were not long, so simple tenses were used. Past Perfect had done indicates earlier action, Past Simple started describes later action.
After I had been doing excersise, I was to tired to write an essay.
Here the situation is simmilar to the previous sentence. The only difference is that the first action is longer which is indicated by using Past Perfect Progressive tense had been doing