Read the first two examples by naming each part.
إِنْ تَجْلِسْ أَجْلِسْ
Condition: if you sit. Answer: I will sit.
إِنْ تَضْرِبْنِي أَضْرِبْكَ
Condition: if you strike me. Answer: I will strike you.
In تَضْرِبْنِي, the attached نِي means “me.” The jussive sukun remains on بْ before that suffix.