Final answer:
In the sentence 'They kissed the baby goodnight,' 'goodnight' serves as a complement and 'the baby' is the direct object. When converting to passive voice, the correct form is 'The baby was kissed goodnight by them.' 'Goodnight' cannot act as a direct object and thus, the other suggested passive phrase is incorrect.
Step-by-step explanation:
In the sentence "They kissed the baby goodnight," the word 'kissed' is the main verb, 'the baby' is the direct object of the verb, and 'goodnight' functions adverbially. Essentially, 'goodnight' is what is known as a complement; it completes the action of saying farewell to the baby in the context of kissing. So, when converting to passive voice, the correct form would be: "The baby was kissed goodnight by them," where 'the baby' becomes the subject, 'was kissed' is the passive verb, and 'goodnight' remains in its role as a complement. The phrase "Goodnight was kissed for the baby by them" does not make grammatical sense, as 'goodnight' cannot serve as a direct object.
Therefore, the use of passive voice here maintains the importance of the action performed on the baby, with less emphasis on who performed the action. The prepositional phrase in the passive sentence is 'by them,' indicating the doers of the action. The attempt to direct the sentence's focus towards the baby is consistent with guidelines suggesting preference for passive voice when the object being acted upon is more significant than the doer.