LEARN KOREAN (한국어) [GRAMMAR] #1

ㅡ IRREGULAR

If the final letter in a stem is ㅡ (for example: 잠그다 - to lock), when adding ~아/어, you can not determine whether you need to add ~어 or ~아 to the stem by looking at ㅡ. Instead, you must look at the vowel in the second last syllable. For example, in the word 잠그다, the second last syllable in the stem is 잠, and the vowel here is ㅏ. We use the same rules as above to determine whether 아 or 어 is added. If the vowel in the second last syllable is ㅏ or ㅗ, 아 is added. If the vowel in the second last syllable is anything other than ㅏ or ㅗ, 어 is added. For example:
Where the vowel in the second last syllable is ㅏ or ㅗ:
잠그다 + ~아/어
= 잠그아

Where the vowel in the second last syllable is anything butㅏ or ㅗ:
슬프다 + ~아/어
= 슬프어

Unfortunately, that is not all that happens. When ~아/어 is added to a stem where the last letter is ㅡ, ~아/어 merges to the stem and the ㅡ is eliminated. That is quite confusing, so let's look at examples of how this is done.

잠그다 - to lock
This stem ends in ㅡ. If we were to add ~아/어, we need to look at the second last syllable, which is 잠. The vowel in this syllable is ㅏ, so ~아 needs to be added to the stem.
잠그 + 아
~아 merges to the stem and the ㅡ is eliminated:
잠그 + 아 = 잠가

예쁘다 = pretty
This stem ends in ㅡ. If we were to add ~아/어, we need to look at the second last syllable, which is 예. The vowel in this syllable is not ㅏ or ㅗ, so ~어 needs to be added to the stem.
예쁘 + 어
~어 merges to the stem and the ㅡ is eliminated:
예쁘 + 어 = 예뻐

This merging of ~아/어 to the stem occurs when ~아/어 is added, or any of its derivatives, including ~아/어요 and past tense conjugations like ~았/었어 or ~았/었어요. For example:

저는 문을 잠갔어요 - I locked the door
저는 바빠요 - I am busy
그 여자는 예뻐요 - That girl is pretty

Some stems only have one syllable. For example, the stem of 크다 is just 크. In this case, we know that we need to use the ㅡ irregular, but there is no previous syllable to draw on to determine what should be added to the stem. In these cases, ~어 is added to the stem. For example:

크다 - to be big
This stem ends in ㅡ. If we were to add ~아/어, we need to look at the second last syllable, but there is none, so ~어 needs to be added to the stem.
크 + 어
~어 merges to the stem and the ㅡ is eliminated:
크 + 어 = 커
그 집은 커요 - That house is big

Sometimes the last vowel of a stem is ㅡ, but the stem ends in a consonant. In these cases, there is no merging and ~어 is always chosen instead of ~아. For example:

긁다 - to scratch
The last vowel in the stem is ㅡ. However, the stem does not end in ㅡ and instead ends with a consonant. Therefore, 어 is added.
긁 + 어요 = 긁어요
저는 머리를 긁었어요 - I scratched my head

듣다 - to hear
The last vowel in the stem is ㅡ. However, the stem does not end in ㅡ and instead ends with a consonant. Therefore, 어 is added.
듣 + 었어요 = 듣었어요

But wait! Don't forget the ㄷ irregular.

듣 + 었다 = 들었다
저는 쥐를 들었어요 - I heard a mouse

만들다 - to make
The last vowel in the stem is ㅡ. However, the stem does not end in ㅡ and instead ends with a consonant. Therefore, 어 is added. Regardless of what the vowel in the previous stem is, because the stem ends in a consonant, 어 is added.
만들다 + ~아/어요
= 만들어요

Here is a table with the word "잠그다 (to lock – which is a verb)" being conjugated using all the honorific forms you have learned so far. The irregular conjugations are in bold. Notice that this only occurs when ~아/어 (or one of its derivatives like ~았/었어요) is added to the verb stem.

And here is a table with the word "예쁘다 (pretty – which is an adjective)" being conjugated using all the honorific forms you have learned so far. The irregular conjugations are in bold. Notice that this only occurs when ~아/어 (or one of its derivatives like ~았/었어요) is added to the verb stem.

Finally, here is a table with the word "만들다 (to make – which is a verb)" being conjugated using all the honorific forms you have learned so far.