Postverbal al-bābu is passive subject; preverbal al-bābu is a nominal subject whose predicate is a passive verbal clause with an implied subject.
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Fronted Noun versus Postverbal Subject
Comprehensive Review: Applied Arabic Parsing
Postverbal al-bābu is passive subject; preverbal al-bābu is a nominal subject whose predicate is a passive verbal clause with an implied subject.