You already know the 6 basic vowels. Now, four of them get a "Y" upgrade! The rule is beautifully simple: adding one extra stroke to a basic vowel turns it into its Y-version. ㅏ becomes ㅑ, ㅓ becomes ㅕ, ㅗ becomes ㅛ, and ㅜ becomes ㅠ. Notice that ㅡ and ㅣ don't have Y-versions -- they play a different role in compound vowels later.