Here's the list of the vowels that make ㅅ sound as "sh": 시 = shi 쉬 = shwi 샤 = sha 셔 = shŏ 쇼 = sho 슈 = shu 섀 = shae 셰 = she You might also hear people pronounce 쉐 as 셰 but it's wrong. 쉐 is swe, no "sh" sound.

Another way to explain it is this: s + i = shi So like Hyejong said, there's "si" which becomes "shi". For the other ones, I understand the second "vowel bar" as meaning "+ y", which effectively turns them into "sha", "sho", etc. (but it's not the case with other consonants, only with the "s"). I don't know which explanation is easier to understand for you! ^^