In regular Form I second-person active commands, kasrah is used on the initial connecting hamzah when the middle root consonant (ʿayn al-kalimah) carries fatḥa or kasrah in the imperfect. “Middle root consonant” is a morphological position, not merely the second written letter.
Word / formMeaning / explanation
تَضْرِبُ → اِضْرِبْstrike; rāʾ takes kasrah.
تَفْتَحُ → اِفْتَحْopen; tāʾ takes fatḥa.
تَسْمَعُ → اِسْمَعْlisten; mīm takes fatḥa.
The initial kasrah belongs to hamzat al-waṣl; the root and its imperfect pattern remain distinguishable.