The underlying root of رَضِيَ is ر ض و. Its perfect shows yāʾ through the weak-final rule, while the imperfect shows alif maqṣūrah.
CategoryFormMeaning
Active perfectرَضِيَhe was content
Active imperfectيَرْضَىhe is content
Passive perfectرُضِيَhe was approved of
Passive imperfectيُرْضَىhe is approved of
Active participleرَاضٍcontent, approving
Passive participleمَرْضِيٌّaccepted, pleasing
Imperativeاِرْضَbe content!
Prohibitionلا تَرْضَdo not accept/be content!
In the command and prohibition the final alif maqṣūrah of the imperfect-based pattern is elided, leaving fatḥah.