1) All the breakfast had been eaten before the conversation was finished. SUBJECT + PAST PERFECT + BEFORE + NOUN + PAST SIMPLE + NO AGENT 2) About four-fifths of the City was consumed by the Great Fire of London in 1666. SUBJECT + PAST SIMPLE + AGENT + TIME 3) Photographs were being taken during the ceremony. SUBJECT + PAST CONTINUOUS + DURING + NOUN + NO AGENT 4) The Queen will be met by the student delegation at the airport. SUBJECT + FUTURE SIMPLE + AGENT + PLACE 5) All main courses are served with salad or seasonal vegetables. SUBJECT + PRESENT SIMPLE + NOUN + NO AGENT 6) Lessons ought to be made more interesting for young children. SUBJECT + MODAL VERB + CLAUSE + NO AGENT
The object of the active sentence becomes the subject of the passive sentence.
BY + AGENT can be omitted when the agent is unimportant, unknown, obvious from the surrounding context or words like: somebody, someone, nobody, you, she, they, people etc. They are rebuilding the house now. => The house is being rebuilt now by them. Somebody has given her twenty roses. => Twenty roses have been given to her by somebody.