ضَرَبْتُ زَيْدًا رَأْسَهُ — I struck Zayd’s head.
ضَرَبْتُ: past verb with subject -tu.
زَيْدًا: accusative object, antecedent.
رَأْسَهُ: part-for-whole badal, accusative with fatḥah; raʾs is mudāf and -hu mudāf ilayh.
-hu: pronoun referring back to Zayd.
The book parses raʾsahu as badal, not as an independent second object of ḍaraba.