Tense              Subject           Verb              Object
writes        a letter.
                 Active:          Rita
Simple Present
                 Passive:      A letter     is written      by Rita.
                 Active:          Rita        wrote         a letter.
 Simple Past
                 Passive:      A letter    was written      by Rita.
                 Active:          Rita      will write      a letter.
    Future
                 Passive:      A letter   will be written   by Rita.