웃다 conjugated in the past tense is 웃었어요
There is a rule that says if the verb stem ends in a ㅅ then you drop the ㅅ if the conjugation being added starts with a vowel. (For example, 짓다 -> 지었어요)
Why is 웃다 an exception? How do you recognize that a word is an exception to this rule?
ㅅ 불규칙 활용 (ㅅ irregular usage)
낫+아->나아 (adj; 그는 병이 나아졌다. His body in state of sickness was better)
긋+어 -> 그어 (draw a line)
붓+어 -> 부어 (그는 풍선처럼 얼굴이 부었다 His face was larger like a balloon)
동사는 벗다 솟다 씻다 뺏다 웃다 등을 제외하면 대부분 불규칙이다.
Almost all verbs except take off clothes, rise, wash, steal, smile are irregular
형용사는 낫다를 제외하면 모두 규칙이다.
All adjectives except heal are regular
reason : 중세국어는 지금에 안쓰는 음가를 사용했다. 그것이 사라진 것이다.
Korean of Middle ages contains some sound not used in this time. But nowadays we do not use so that ㅅ is disappeared.
there really aren't that many verb roots ending in ㅅ and here are the main (if not the only) ones that are irregular:
Affected Verbs: 긋다, 낫다, 붓다 (들어붓다, 퍼붓다), 잇다, 잣다, 젓다 (휘젓다), 짓다 (매듭짓다)
any others not on this list (or compound verbs ending with these) are not irregular.
