Learn Hardcore Korean: haneuri malgayo.

하늘 means “sky.” The particle 이 marks it as the subject of the sentence. You use 이 after a noun ending in a consonant (and 가 after a noun ending in a vowel).

What is 맑아요, and how is it formed?

맑아요 is the polite present-tense form of 맑다, a descriptive verb meaning “to be clear” or “to be bright.” To form it, you:

Drop the dictionary-ending 다 from 맑다.
Since the last vowel in 맑다 is ㅏ, you add -아요 to get 맑아요.

What part of speech is 맑다, and how does it behave?

맑다 is a descriptive verb (often called an adjective in English grammar). Descriptive verbs in Korean conjugate like action verbs and always come at the end of a clause.

Why is -아요 used instead of -어요, and how do I choose?

Korean attaches -아요 or -어요 based on the vowel in the verb stem:

If the last vowel is ㅏ or ㅗ, use -아요.
For all other vowels, use -어요.
Since 맑 has the vowel ㅏ, we use -아요.

What formality level is 맑아요?

맑아요 is the polite (해요) style, appropriate for everyday conversation with strangers or acquaintances. It’s neither too formal nor too casual.

Could I say 하늘이 맑습니다 instead?

Yes. 맑습니다 is the formal (합니다) style. Use it in presentations, news reports, or when speaking to someone much older or in a very formal setting.

How would I make the sentence negative or turn it into a question?

Negative: 하늘이 맑지 않아요.
Question (polite statement with rising intonation): 하늘이 맑아요?
Or question using -습니까? in formal style: 하늘이 맑습니까?

Can the subject 하늘이 be omitted?

Often in Korean, if context is clear, you can drop the subject. Simply saying 맑아요. can imply “It’s clear” (referring to the sky) when talking about the weather.

What’s the difference between 맑아요 and 맑네요?

맑아요 states a fact politely.
맑네요 adds the speaker’s impression or surprise: “Oh, the sky is so clear (isn’t it)!” It’s still polite but conveys emotion or new information.