Answer:
First question is an example of how the rest should be answered.
2. I'm making soup and the dessert was prepared by Amy.
When an event was in the past, the passive form would be the word "was" for a singular subject and the past participle of the verb used (prepare) which in this case is "prepared".
3. The restaurant will be opened by the mayor tomorrow.
For future events, the passive form would be the phrase "will be" and then the past participle of the verb (open) in question which is "opened".
4. The reception starts soon and the cake has not been delivered yet!
For ongoing events, the passive form is a present perfect tense which would take the form of "has not been" for singular subjects in addition to the past participle of the verb (deliver) which is "delivered".