In Michael Swan and Carherine Walter book "how ENGLISH works" there are exercises:
Choose the right tenses (present perfect, past or past perfect, simple or progressive).
25. After he (finish) breakfast he (sit) down to write some letters.
My answer is :
finished; has been sitting;
but correct answer in the book is:
had finished; sat;
I am thinking that both are correct and ask myself why authors did not give us both solution like correct? My thinking I have just shared with you.