Pick out the sentences in past indefinite passive: 1. dissecting the corpse the students separated one muscle from the other. 2. on the corpse one muscle was separated from the other by the students. 3. the doctor was treated by many specialists as he was badly ill himself. 4. the doctor treated his patients successfully

2 and 3
Медицинский вуз, да?

Признаки Пассивного залога: глагол to be (are, is, was. were, has/have been) + смысловой глагол в 3 форме (правильные - основа+ed, неправильные - по таблице)
Смысл Пассивного залога: с первым словом в предложении кто-то что-то делает.