곶[串] is an interesting case as it’s the only example I can think of where a Hanja is used with a sound value that is definitely not borrowed from Chinese. In fact, searching a bit revealed this site which tells quite an interesting history: The character 串 (Sino-Korean reading 천 or 관) means “string of objects” (in fact it depicts a two items strung together) or also “skewer”. The Korean word for “skewer” is 꼬치, coming from the verb 꽂다 “to stab, to stick into” + a noun suffix. So when Hangeul wasn’t as widespread people used the character 串 to write both 꼬치 and 꽂다 – or at least sort of because in Middle Korean times those words actually didn’t have a tense initial yet; they were 고지 and 곶다. In other words, 串 was used to write a homophone to the word 곶 “cape, landmass protruding into the water”, so it was just borrowed to write that word as well – and for some reason stuck around with that meaning.
In English, present progressive is used to describe a temporary, volatile state of things. Present simple, permanent state.
In Korean, present simple is used regardless of the variant nature of things. Korean Present progressive is usually used for emphasis purpose, to put stress on the importance of a situation developing "right now".
For those who might need it,
삶다 - to boil something inside the liquid. Eg- boiling the eggs
끓다 - to boil a liquid (used when there is no mention of the person doin it). Eg- the milk is boiling.
끓이다 - to boil something (when there is a mention of a person doing it). Eg - mom is boiling the milk
끓나다 - something has boiled and is overflowing (coming out of the vessel). Eg- the milk is boiling out of the vessel.
• 끓다 = to boil / be boiling (where "boiling" is a gerund) / be at boiling point
끓다 describes the state of the subject. It is a passive hence an intransitive verb
우유가 끓습니다. The milk boils or is boiling / is at boiling point
• 끓이다 =
(i) to make (something) boil; or cause (something) to boil; or to boil/be boiling (something) (where boiling is the present participle of "to boil" and describes an action in progress.)
(ii) to be boiled by
끓이다 can be both causative and passive
As causative verb (i), it is a dynamic, transitive verb i.e. it can have a direct object.
어머니가 우유를 끓입니다 - Mother makes the milk boil, or Mother boils/is boiling the milk
As passive verb (ii), it is an Intransitive verb and cannot have a direct object complement.
우유가 어머니가 끓입니다. The milk is boiled by mother.