Vowel Base Verbs

Bases ending in ㅏ,ㅓ,ㅐ merge with verb endings that start with 아 or 어.
자다 --> 자 (to sleep --> sleeps).
If the base ends in ㅣ and an ending starting with 어 is added, both vowels merge to form ㅕ.
가르치다 --> 가르쳐 (to teach --> teaches)
Note that 위 and 어 cannot be merged because this syllable does not exist.
If the base ends in ㅜ and the ending starting with 어 is added, both vowels merge to form ㅝ.
배우다 --> 배워 (To learn --> learns)
If the base ends in ㅗ and the ending starting with 아 is added, both vowels merge to form ㅘ.
오다 --> 와 (To come --> comes)
Most vowel bases that end in ㅡ drop the ㅡ beffore adding endings that start with vowels. (어 is added except for verbs with a preceding syllable with ㅗ or ㅏ)
크다 --> 커 (to be big --> is big)
바쁘다 --> 바빠 (to be busy --> is busy)
Most vowel bases that end in 르 drop the ㅡ and double the ㄹ before adding verb endings that start with a vowel. (어 is added except for verbs with a preceding syllable with ㅗ or ㅏ)
빠르다 --> 빨라 (To be fast --> is fast)

Grammar