The person and number stay the same when an eligible Light-Noon form changes from active to passive.
PassiveActivePosition
لَيُفْعَلَنْلَيَفْعَلَنْmasculine Ghaa'ib singular
لَتُفْعَلِنْلَتَفْعَلِنْfeminine Haadhir singular
لَنُفْعَلَنْلَنَفْعَلَنْMutakallim plural
First identify the position, then use the vowel pattern to identify the voice.