Learn Hardcore Korean: cheonggyeori jungyohaeyo.

청결: a Sino-Korean noun (from 清潔) meaning cleanliness, the condition/state of being clean. Slightly formal/technical; common in rules, notices, and business contexts.
청소: the act of cleaning. Verb form is 청소하다.
깨끗함: the abstract noun formed from 깨끗하다 (clean). More everyday in tone, but as a subject it’s less common than using 청결 in formal statements.
위생: hygiene; health/sanitation-focused. Often used in medical, food service, or public health contexts.

Why is the subject particle 이 used after 청결, and when would I use 가 or 은/는 instead?

이 follows a noun ending in a consonant; 가 follows a noun ending in a vowel. 청결 ends in the consonant ㄹ, so 청결이 is correct.
이/가 marks the grammatical subject and often highlights new or focused information.
은/는 is the topic/contrast marker. It frames what you’re talking about or contrasts it with something else.

Can I say 청결은 중요해요 instead? What’s the nuance difference from 청결이 중요해요?

청결이 중요해요 answers a question like 뭐가 중요해요? by focusing the subject: Cleanliness (as the thing under discussion) is what’s important.
청결은 중요해요 sets up a topic or a contrast: As for cleanliness, it’s important. For example: 맛도 중요하지만, 청결은 더 중요해요.

Why is it 중요해요 and not something like 중요이에요? How does 중요하다 conjugate?

중요하다 is the dictionary form. With the polite -아요/어요 ending, 하다 → 해요, so you get 중요해요.
이다 attaches to nouns (e.g., 학생이에요). While 중요 is a noun meaning importance, to say something is important you use 중요하다, not 중요이다. So 중요이에요/중요예요 is not used for this meaning.

Is 중요하다 a verb or an adjective in Korean?

It’s a descriptive verb (형용사). It functions like an adjective in meaning but conjugates like a verb:

Predicate: 중요해요, 중요합니다, 중요했어요, 중요할 거예요
Modifier: 중요한 (e.g., 중요한 문제)
Adverbial: 중요하게 (e.g., 중요하게 생각하다)

What politeness level is 중요해요? How do I make it more or less formal?

중요해요: polite informal (해요체), default in most conversations.
중요합니다: formal polite (합니다체), common in presentations, announcements, writing.
중요해: casual (반말) to friends or younger people.
Dictionary/citation form: 중요하다.

How do you pronounce the whole sentence naturally?

청결이: the final ㄹ links to the following vowel, so it sounds like [청겨리] (cheong-gyeo-ri).
중요해요: pronounced [중요해요] (jung-yo-hae-yo). There’s no n-insertion; it’s not [중뇨…].
Official RR romanization: cheonggyeol-i jungyohaeyo. Actual sound: cheonggyeori jungyohaeyo.

Can I drop the subject and just say 중요해요?

Yes. Korean frequently omits obvious subjects. In context, 중요해요 will be understood as It’s important. Use 청결이 when you need to be explicit or contrast with other possibilities.

How can I make the statement stronger or more emphatic?

Add intensifiers: 아주/매우/정말/진짜/굉장히/엄청 → 청결이 정말 중요해요.
Superlative: 가장/제일 → 청결이 가장 중요해요.
Focus: 특히/무엇보다 → 청결이 무엇보다 중요해요.

How do I express comparisons like more important or most important?

Comparative with 더 (more) and 보다 (than): 맛보다 청결이 더 중요해요.
Superlative with 가장/제일: 청결이 제일 중요해요.
Less important: 덜 → 청결이 덜 중요해요. (less common, but grammatical)

What verbs or collocations commonly go with 청결?

청결을 유지하다 (maintain cleanliness)
청결을 지키다 (keep cleanliness)
청결 관리 (cleanliness management)
개인 청결 (personal cleanliness), 주방 청결, 매장 청결, 환경 청결
Explanatory follow-ups: 청결이 중요하니까 청결을 유지해야 해요.

Is 청결이 중요해요 interchangeable with 위생이 중요해요 or 깨끗함이 중요해요?

위생이 중요해요 leans toward hygiene/health standards (medical, food safety).
청결이 중요해요 is broader and a bit formal, good for rules or policies.
깨끗함이 중요해요 sounds more everyday but is used less often in set phrases; many speakers would instead say 깨끗한 게 중요해요. They overlap, but choose based on context and register.

Can I add the honorific -시- here, like 중요하세요?

No. -시- honors an animate subject (a person of higher status). 청결 is an abstract concept, so 청결이 중요하세요 is ungrammatical. Use 중요하십니다 only when the honored subject is a respected person (or tightly associated with one), which doesn’t apply here.

How do I use 중요하다 before a noun or turn it into a noun?

Attributive modifier: 중요한 → 중요한 역할, 중요한 포인트
Nominalizations: 중요함, 중요성 (importance; 중요성 is more formal/technical)
Adverbial: 중요하게 → 이 문제를 중요하게 다뤄요.

Any spacing or spelling points to watch?

Particles attach without a space: 청결이, not 청결 이.
Verb endings attach without a space: 중요해요, not 중요 해요.
The sentence is two spacing units: 청결이 중요해요.