Learn Hardcore Korean: seojjok haneure gureumi manhayo. - There are many clouds in the western sky.

서 means ‘west.’ Adding 쪽 (literally ‘side’ or ‘direction’) to a cardinal point turns it into a noun meaning ‘the west side’ or ‘western.’ So 서쪽 = ‘western.’

What does the particle 에 do in 서쪽 하늘에?

에 is the locative particle meaning ‘in,’ ‘at,’ or ‘on.’ Here it marks 서쪽 하늘 (‘western sky’) as the place where something exists: ‘in the western sky.’

Why is 구름 followed by 이 instead of 은?

이/가 are subject markers; they tell you what’s doing or being described. Since 구름 (‘clouds’) is the subject of 많아요, you use 이 (after a consonant). 은/는 are topic markers and would add a contrasting or general-topic nuance.

What part of speech is 많다 and how does it function in 구름이 많아요?

많다 is a descriptive verb (often called an adjective in English). In Korean grammar, adjectives are conjugated like verbs. 많아요 is its polite-present form, meaning ‘are many’ or ‘be plentiful.’

What’s the difference between 구름이 많아요 and 구름이 많이 있어요?

구름이 많아요 uses the descriptive verb 많다 directly: ‘Clouds are many.’
구름이 많이 있어요 uses 많이 (the adverb ‘a lot’) + 있다 (‘to exist’): ‘There are a lot of clouds.’
Both express the same basic idea; the first emphasizes the quality 많다, the second emphasizes existence with quantity.

Can I say 서쪽 하늘에 많은 구름이 있어요 instead?

Yes. 많은 is the attributive (modifying) form of 많다, so 많은 구름 = ‘many clouds.’
서쪽 하늘에 많은 구름이 있어요 = ‘There are many clouds in the western sky.’ It sounds a bit more formal or literary but means the same thing.

What nuance does 서쪽 하늘에는 (instead of 서쪽 하늘에) add?

에는 = locative 에 + topic 는. It sets 서쪽 하늘 up as the topic, adding a subtle contrast or emphasis:
‘As for the western sky, there are many clouds (perhaps unlike other skies).’

Is 구름이 서쪽 하늘에 많아요 also correct? How flexible is the word order?

Korean word order is flexible, but the neutral order for a locative description is 서쪽 하늘에 구름이 많아요 (location → subject → predicate).
Putting 구름이 first isn’t wrong, but sounds less natural: Koreans usually mention the place before the subject in such sentences.

How do I pronounce 서쪽 하늘에 구름이 많아요?

A simple romanization is seo-jjok ha-neul-e gu-reum-i man-a-yo.
In IPA: [sʌt̚.t͡ɕʰa.nɯ.le̞ ɡu.ɾɯ.mi man.ʰa.jo̞].

How can I change the politeness level of 서쪽 하늘에 구름이 많아요?

Formal polite/written: 서쪽 하늘에 구름이 많습니다.
Informal polite (to friends): 서쪽 하늘에 구름이 많아.
Very casual (slangy): 서쪽 하늘에 구름 진짜 많아!