1. Your shoelace is loose. (WRONG)
2. Your shoelace is untied. / undone.
3. Your shoelace came undone.
untie / undo
I never undo my shoelaces when I take off my trainers.
undo（及物動詞)意思是「解開」， 它的用法很多, 字義除了有「鬆綁」之外，以下情況均可適用:
1. 解開鈕扣 (undo the buttons)
2. 拉開拉鍊 (undo the zipper)
3. 脫下項鍊 (undo the necklace)
4. 解開腰帶 (undo the belt)
5. 拉開門閂 (undo the latch)
6. 轉開螺絲 (undo the screw)