|13.6.4 exercise2: The construction stare + gerundio|
1) The Italian imperfect can be used to describe a basic action which has been interrupted by another action.
a) I was reading a book when suddenly someone knocked at the door.
b) Leggevo un libro quando di golpo qualcuno bussò alla porta.
c) Stava leggendo un libro quando di golpo qualcuno bussò alla porta.
In the English sentence the use of the continuous form is compulsory, the simple past can't be used in this context. But Italian have already a tense to describe ongoing actions that were interrupted by another action, the imperfetto. So the use of the construction stare + gerundio is not compulsory. It can be used, it is grammaticaly correct, but not compulsory and normally it woudn't be used in this context. An English native speaker tends to use the construction stare + gerundio in this context because in English he would use the continuous form, but it is not compulsory in Italian in this context.
2) The English present perfect continuous and the English past perfect continuous are used to describe an action which started in the past and continued or stopped immediately before another actions started.
He had been looking for his keys the whole morning, when he found them under his clothes.
I have been working the whole day on it and it is not finished.
In Italian you can use the construction stare + gerundio in this context, but it is not compulsory.
Ho lavorato tutto il giorno, sono stanco adesso.
Ho stato lavorando tutto il giorno, sono stanco adesso.
I have been working the whole day, I am tired now.
An English native speaker tends to use the construction stare + gerundio every time he would use the gerund in English. Chose in the following sentences the plausible italian translation.