In Session 4, a lot of the beginning chapters are about past participles. In the lessons it says that for certain verbs, if they are followed by another verb, you would switch using Avere to Essere. These verbs are Dovere, potere, and volere.
1) Siamo dovuti partire - We had to leave 2) Non ha voluto mangiare - (he) didn't want to eat.
Why did the first sentence change to Essere and not the 2nd sentence?