How do we use perfect tense in passive voice?

Answer 1

The perfect isn't so much a tense as it is the formula: haben/sein + past participle. The passive is also not really a voice, but instead the formula: werden + past participle [+ von]. The only problem is that when werden is used as a past participle in the perfect, it goes from 'geworden' to 'worden' for no good reason. So when you combine the two formulas you get something like: The pizza has been eaten. – Die Pizza ist gegessen worden.