When essere is the auxiliary, the past participle must agree: -o (m.sg.), -a (f.sg.), -i (m.pl. or mixed), -e (f.pl.). Wrong agreement is a common error.
Congiuntivo Passato — essere verbs: agreement (Group 2)1 / 10
Penso che i ragazzi ___ (tornare). [ragazzi = m.pl.]