Plural objects help us see why “mansub” does not always mean a visible fathah.
كَتَبَ الْكُتُبَ — he wrote the books.
- the understood “he” is the fa'il;
- الْكُتُبَ is a broken-plural maf'ul bihi, mansub with fathah.
خَلَقَ اللَّهُ السَّمَاوَاتِ — Allah created the heavens.
- اللَّهُ is the fa'il;
- السَّمَاوَاتِ is a sound-feminine-plural maf'ul bihi, mansub with kasrah.