ㅅ irregular verbs
Some verbs and adjectives whose stem ends with a ㅅ 받침 are irregular.
When these irregular stems are followed by a vowel, the ㅅ is dropped.
Note that some verbs/adjectives with a ㅅ 받침 are regular: the ㅅ does not change. There is no specific rule to know whether it is an irregular verb or not; instead you must memorize which are irregular verbs and which ones aren't.
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Some irregular ㅅ verbs are: 낫다, 짓다, 긋다, 잇다.
Some regular ㅅ verbs (which do not drop the ㅅ 받침) are: 벗다, 웃다, 빗다, 씻다, 빼앗다.