How do I change 'The teacher punished the students' into passive voice?

In the active sentence, 'The teacher' is the subject, 'punished' is the verb, and 'the students' is the object. To form the passive, make the object the new subject: 'The students'. Then use the correct form of 'be' in the same tense plus the past participle of the verb. The tense here is simple past, so use 'were' (plural subject) plus 'punished'. Finally, add 'by' before the original subject. The passive sentence is: The students were punished by the teacher. Always keep the tense the same, match 'be' to the new subject's number, and use the third form of the verb. For simple present you would use is/are, and for future you would use 'will be'.