LEARN KOREAN (한국어) [GRAMMAR] #1

ㅅ IRREGULAR

If the last letter of a word stem ends in ㅅ (for example: 짓다 - to build), the ㅅ gets removed when adding a vowel. For example, when conjugating:

짓다 - to make/build
짓 + 어 = 지어
나는 집을 지어 - I build a house

짓 + 었어요 = 지었어요
저는 집을 지었어요 - I built a house

Notice that this only happens when adding a vowel. When conjugating to the plain form, for example, you only add "는다" to a stem and thus ㅅ does not get removed. For example:

집을 짓는다 - to build a house

The reason this irregular is done is to avoid changing the sound of a word completely after conjugating it.

Pronouncing 짓다 sounds like 'jit-da.'
Pronouncing 지어 sounds like 'ji-uh.'
Pronouncing 짓어 sounds like 'jis-suh.'

The third one (which is incorrect) completely changes the sound of the word stem when a vowel is added (from 'jit' to 'jis.') Whereas in the second one, the sound of the word stem only changes from 'jit' to 'ji,' which is much smaller of a difference (especially considering the 't' in the pronunciation of 짓 is not aspired – which makes it barely audible). I know that is confusing, but if you can't understand why it is done, that's fine. Just know that it must be done.

Some other examples of words that follow this irregular are (these words are too difficult for you right now, but I'm just showing you):

낫다 - better (adjective)
잇다 - to continue (verb)

Common words that this does not apply to are:
웃다 (to laugh)
저는 웃었어요 - I laughed

벗다 (to take off clothes)
저는 저의 옷을 벗었어요 - I took off my clothes

씻다 - to wash
저는 저의 손을 씻었어요 - I washed my hands

Here is a table with the word "짓다 (to build)" 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.