ㅎ irregular verbs
From Korean Wiki Project
Some verbs and adjectives whose stem ends with a ㅎ 받침 are irregular.
These irregular verbs/adjectives drop the ㅎ and conjugate like stems without 받침 in some cases (see below), and use the 야 conjugation in A/V + 아 / 어 / 여 forms.
Note that many verbs/adjectives with a ㅎ 받침 are regular (the ㅎ is not dropped, etc.). There is no specific rule to know whether one is irregular or not; instead you must memorize which are irregular and which ones aren't.
This section is incomplete and needs to be filled in.
Some irregular ㅎ verbs/adjectives: 노랗다, 빨갛다, 까맣다, 하얗다, 파갛다, 그렇다, 어떻다
Some regular ㅎ verbs/adjectives (which do not drop the ㅎ): 좋다, 넣다, 놓다, 많다, 낳다, 찧다, 쌓다, 싫다, 괜찮다