أَعْجَبَنِي زَيْدٌ عِلْمُهُ — Zayd impressed me—his knowledge did.
أَعْجَبَ: past verb; -nī is its object.
زَيْدٌ: nominative subject and antecedent.
عِلْمُهُ: nominative badal al-ishtimāl; ʿilm is mudāf and -hu mudāf ilayh.
The book’s marginal commentary gives this explanatory example. Knowledge is associated with Zayd, not a physical part of him.