줄다: 'something shrinks' as an intransitive verb 옷(clothes)이 줄었어요(shrank). 너무(too) 작아서(because it is small) 못 입어요(can't wear). 줄이다: 'reduce something' as a transitive verb 옷(clothes) 사이즈(size)를 줄였어요(I reduced the size of the clothes).