Irregular ㅡ
For verb and adjective stems that end in ㅡ, ㅡ is omitted without exception when adding an ending that begins with 아/어. Then, the vowel that preceded ㅡ determines whether 아 or 어 is used in the ending. That is, when the vowel is ㅏ or ㅗ, ㅏ is used, otherwise ㅓ is used. In the case of single-syllable stems in which ㅡ is the only vowel, ㅓ is added after ㅡ is omitted.
For verb and adjective stems that end in ㅡ, ㅡ is omitted without exception when adding an ending that begins with 아/어. Then, the vowel that preceded ㅡ determines whether 아 or 어 is used in the ending. That is, when the vowel is ㅏ or ㅗ, ㅏ is used, otherwise ㅓ is used. In the case of single-syllable stems in which ㅡ is the only vowel, ㅓ is added after ㅡ is omitted.
ㅡ is always omitted when adding an ending that start with 아/어.
ㅡ is always omitted when adding an ending that start with 아/어.