Active- The firemen took the injured to the hospital.
Passive- The injured were taken to the hospital by the firemen.
There are two basic rules for converting sentences from Active Voice into Passive Voice, which are common for all tenses-
1) The places of subject and object will be interchanged in the sentence.
Active voice- Sub. + have/has + P.P.form of verb + obj.
Passive voice- Sub.(obj.) + have been/has been + P.P. form of verb + prepositional by + obj.(Sub.)
2) Only the 3rd form of the verb or Past Participle (e.g. written) will be used as a main verb in Passive Voice.