How does "I don't know" work here? I don't understand
أَعْرِف I know.
So together they mean something like "I know not" (which may sound stilted today but was once a common way to say "I don't know" in English).
Is Bob the subject of the verb here? Or is Bob the person the speaker is talking to, in which case a better grammatical translation would be "I don't know his friend, Bob? This has been confusing me in these lessons.
"I" is the subject of the verb. Bob equals the friend that you do not know. Bob is the direct object of the English sentence.