الفاعل مرفوع، والمفعول منصوب، والمضاف إليه مجرور.
The grammatical subject takes the nominative, the object the accusative, and the genitive construction's second noun the genitive.
These are familiar principles among grammarians:
“جَاءَ زَيْدٌ” — Zayd is the nominative subject.
“رَأَيْتُ زَيْدًا” — Zayd is the accusative object.
“كِتَابُ زَيْدٍ” — Zayd is genitive as the second term of annexation.
Calling the rules widely accepted in a discipline does not imply they lack evidence. Their disciplinary acceptance makes them useful common starting points.