Verbs and adjectives conjugate in Korean. The present tense conjugation of the polite (common) form:
This is what Rob Julian of KDA calls the clock theory.
I prefer to think of it as described in “Integrated Korean”. All vowels are composed of three strokes heaven (-), earth (ㅡ), and human (ㅣ). Vowels are bright (ㅏㅗ), dark (ㅓㅜ), or neutral (ㅣㅡ). Traditionally, Korean sought vowel harmony. I find this beautiful.
So stems with bright vowels (ㅏㅗ) get a bright 아+요 when conjugating. The rest get the dark 어+요 ending.
Note: exceptions to this pattern. 이다 ‘to be’ 에요 (or 예요), 아니다 ‘to not be’ 에요.
In all conjugation patterns, when you have a vowel that connects directly to another vowel, they usually contract to something new. See “Mastering Conversational Korean – Korean for Beginners” p. 36 for a chart of the vowel contractions with examples.
|ㅏ + ㅏ||ㅏ|
|ㅓ + ㅓ||ㅓ|
|ㅗ + ㅏ||ㅘ|
|ㅜ + ㅓ||ㅝ|
|ㅡ + ㅓ||ㅓ|
|ㅣ + ㅓ||ㅕ|
|ㅐ + ㅓ||ㅐ|
|ㅔ + ㅓ||ㅔ|
|ㅚ + ㅓ||ㅙ|
|하 + ㅕ||해|
For more information, refer to Talk To Me In Korean Level 1 Lesson 16.