شَكَرَ الطَّالِبُ الْمُعَلِّمَ — The student thanked the teacher.
يَشْكُرُ الطَّالِبُ الْمُعَلِّمَ — The student thanks the teacher.
تَرَكَ حَامِدٌ الْكِتَابَ عَلَى الْمَكْتَبِ — Hamid left the book on the desk.
يَتْرُكُ حَامِدٌ الْكِتَابَ عَلَى الْمَكْتَبِ — Hamid leaves the book on the desk.
The object remains clear in both pairs. The first sentence uses the past form; in the second, context gives the present form a present-time reading.