Focus on two places.
Take يَضْرِبُ: “he strikes.”
1. Change the imperfect prefix from يَـ to يُـ.
2. Give the penultimate root letter a fatḥah: رِ becomes رَ.
3. The result is يُضْرَبُ: “he is struck.”
The action remains imperfect; its voice changes from active to passive.