Passato Prossimo VS. Imperfetto (parte 3ª)

When to use Passato Prossimo and when to use Imperfetto is a common challenge for English-speaking Italian learners.
This final post of the 3-part series will share additional distinctions in using one or the other.

Let’s take a look now at the use of the passato prossimo and imperfetto with modal/helping verbs, which can create quite a bit of confusion as to which to choose.
With the verbs dovere, potere, sapere, and volere it is possible to use both the imperfetto and passato prossimo depending on what is being communicated.

The passato prossimo of modal verbs is used when the result is certain.
Esempi:

Ieri ho dovuto accompagnare mio fratello all’aeroporto.
Yesterday I had to take my brother to the airport.
                                      (I needed to take him and I did, in fact, do so.)
 
Gli ospiti hanno voluto assaggiare il mio limoncello.
The guests wanted to taste my limoncello.
                                     (They wanted to taste it and they did.)
 
Hanno potuto ricevere un rimborso per il biglietto aereo.
They were able to receive a reimbursement for the airplane ticket.
                                     (They had the possibility of getting reimbursed and they were.)
 
Ho saputo gestire la situazione senza problemi.
I was able to manage the situation with no problems.
                                      (I was able to manage it and I did so.)

​In these instances, the listener or reader understands that the desired action was completed or fulfilled. 

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On the other hand, the imperfetto of modal/helping verbs is used when the result of the action is uncertain.
Esempi:

Ieri dovevo accompagnare mio fratello all’aeroporto.
Yesterday I needed to take my brother to the airport.
                            (Whether or not I took him is unknown)
 
Gli ospiti volevano assaggiare il mio vino.
The guests wanted to taste my wine.
                             (Whether or not they tasted it is unknown)
 
Potevano ricevere un rimborso per il biglietto aereo.
They could have received a reimbursement for the airplane ticket.
                             (Whether or not they received it is unknown)
 
Sapevo gestire la situazione senza problemi.
I was capable of managing the situation with no problems.
                             (Whether or not I managed it is unknown)

In the above cases, we might add some information to clarify the circumstances and whether the actions were completed, if it is not already clear:

Ieri dovevo accompagnare mio fratello all’aeroporto, ma la mia auto non aveva la benzina.
Yesterday I needed to take my brother to the airport, but my car was out of petrol.
 
Gli ospiti volevano assaggiare il mio vino, però poi hanno scelto la grappa.
The guests wanted to taste my wine, but then they chose grappa.
 
Potevano ricevere un rimborso per il biglietto aereo e ne hanno approfittato.
It was possible for them to receive a reimbursement for the airplane ticket and they took the opportunity.
 
Sapevo gestire la situazione senza problemi, eppure hanno chiesto altro aiuto.
I was capable of managing the situation with no problems, but still they additional help.