One extra stroke adds a "y": ㅏ → ㅑ (ya), ㅓ → ㅕ (yeo), ㅗ → ㅛ (yo), ㅜ → ㅠ (yu).
ㅐ (ae) and ㅔ (e) sound almost the same today (both like "e" in "bed"). ㅒ (yae) and ㅖ (ye) likewise.
W-vowels combine ㅗ or ㅜ with another vowel:
| ㅘ = ㅗ+ㅏ | wa | 와 |
| ㅙ = ㅗ+ㅐ | wae | 왜 |
| ㅚ = ㅗ+ㅣ | oe (said "we") | 외 |
| ㅝ = ㅜ+ㅓ | wo | 워 |
| ㅞ = ㅜ+ㅔ | we | 웨 |
| ㅟ = ㅜ+ㅣ | wi | 위 |
| ㅢ = ㅡ+ㅣ | ui | 의 |
ㅢ is read "ui" at the start of a word, "i" after a consonant (희 = hi), and "e" as the possessive particle 의.