Word / formMeaning / explanation
يَنْصُرُ ← مَنْصَرٌṣād: ḍammah becomes fatḥah.
يَضْرِبُ ← مَضْرِبٌrāʾ: kasrah stays kasrah.
يَسْمَعُ ← مَسْمَعٌmīm: fatḥah stays fatḥah.
The generalisation is precise: a ḍammah on the imperfect's middle radical changes to fatḥah in this simple place/time noun formation; an existing fatḥah or kasrah is preserved. The prefixed mīm is fatḥah-bearing.