The subject of the predicate (substitute of the subject-doer) نائب الفاعل
It is the grammatical subject of a verb in the passive voice. نائب الفاعل takes the place of the subject-doer when the latter is omitted because the speaker does not know him/her or does not want to name them.
نائب الفاعل should always be in the nominative case raf'a. Ex: كَتَب الطالبُ الدرسَ ← كُتِب الدرسُ
When the subject is omitted, the verb undergoes the following transformations:
- When the verb is in the perfect, then its first letter takes a Damma while its penultimate takes a Kasra. Ex: نُشِرَ الخبرُ/ أُكرِم الضيف
- When the verb is in the imperfect, then the first letter takes a Damma and the penultimate takes a Fatha. Ex: يُنشَر الخبرُ/ يُكْرَمُ الضيفُ
نائب الفاعل can be explicit or implicit. Ex: أُكرِم زيدٌ -أُكرِمتُ.
For a more detailed explanation, see below