Everyone has different leg lengths, so you don't need to worry too much about your knees showing. It could be due to your body shape. However, if your front foot is lifting when you go down, it means your body is unconsciously leaning back. While looking in the mirror, try to widen your legs a bit more than your knees and feel like you're putting weight on your hips as you go down. If you still feel it, try doing it without shoes, and you will understand. Take it slow.

전 앞발이 들리는데 무게중심을 앞으로 놓고 해야 할까요?? 무게중심을 앞으로 두면 무릎이 나와서 운동 자극이 제대로 될 지 모르겠어요ㅠㅠ

If your hind leg is lifted, try to place your body's center a little further back. If you are wearing shoes, take them off and proceed. If it still lifts even without shoes, please step on a thin mat placed in front of each foot as if you are stepping on it.