ㅂ irregular verbs

Question

Korean irregulars are, sometimes, kind of regular, because of rules for conjugation of different patchims. However, I realized today I don't know how to conjugate "regular" irregulars with a ㅂ patchim. Specifically, why do 돕다 and 곱다 get 오, while most others get an 우? I read somewhere that it's because they end in consonant + 오, but that is obviously not true (흥미롭다 -> 흥미로워). One other theory of mine was that it's because the previous syllable (i.e. the one before the ㅂ one) has an 오, so I imagined that because there is no syllable before 돕, for example, it gets this by default... until the obvious realization that this rule would hold true for 고맙다, but it doesn't.

So, are there any rules to adding 오 instead of 우? Or should I just learn these verbs by heart (are there even any other besides 돕다 and 곱다)?

Answer 1

Sources: 한국어의 불규칙 활용
[Grammar] ‘ㅂ’ 불규칙 : ‘ㅂ’ irregular verb/adjective

I tried searching the internet, but 돕다 and 곱다 likely seem to be the only irregularities (of ㅂ irregulars,) and I personally too can't think of any other similar examples. You could view the two as exceptional ones.

Some explanations on the internet claim that this kind of exception occur when the verb stem is single syllable. But I find them not accurate, since we have 깁다, which is conjugated as 기워.

Answer 2

CONSONANT

The ㅂs in those words were actually the 'lighter ㅂ' (여린비읍) in Middle Korean. It sounded like the 'bv' in "obviously"- or the "beta" sound from the IPA. Now, the consonant has died- some merging with the harder ㅂ, some disappearing- and produced irregular conjugations.

VOWEL

Historically, the vowel harmony of Middle Korean made "ㅗ" to be only conjugated with other vowels of similar "feel." Now, the process is dead (in Contemporary Seoul Korean). And the conjugation started to follow other rules like, the style.