Learn Hardcore Korean: eojeneun iri manhaseo jeongmal bappasseoyo. - Yesterday I was really busy because there was a lot of work.

는 is the topic marker. When you say 어제는, you’re framing “yesterday” as the topic—“as for yesterday…,” which can imply a contrast with other days. You could drop it and simply say 어제 일이 많아서…, which is still correct, but using 는 adds a slight nuance of “yesterday in particular.”

Why do we use 많아서 instead of 많았어서 or 많았고?

Korean uses the present stem plus -아서/어서 to express cause, even when the resulting clause is in the past. From 많다 (to be many), remove 다 to get the stem 많-, then add 아서 → 많아서 (“because there was a lot”). You don’t use 많았어서; the past is shown in the main clause 바빴어요.

Could I split the ideas into two sentences: 어제 일이 많았어요. 정말 바빴어요.?

Yes, you can. That gives you two independent clauses: “Yesterday there was a lot of work. I was really busy.” It’s perfectly natural, though linking with 많아서 puts the cause and effect into one smooth sentence.

What level of politeness is 바빴어요, and could I change it?

-았/었어요 is polite informal (standard polite). You could switch to:
• 바빴습니다 (formal polite)
• 바빴어 (casual/informal among close friends)
• 바빴습니다만 (formal with a concession nuance)

Why is 정말 placed before 바빴어요? Can I say 바빴어요 정말?

정말 is an adverb (“really”). Adverbs usually come right before the verb or adjective they modify, so 정말 바빴어요 (“was really busy”) is the norm. Saying 바빴어요 정말 is grammatically understandable but sounds marked or colloquial.

What’s the nuance of using 일이 많아서 instead of 일이 많았어요 + 그래서 바빴어요?

일이 많아서 fuses cause and effect into one clause (“because there was a lot of work…”). 일이 많았어요. 그래서 바빴어요. splits them into two: “There was a lot of work. So I was busy.” Both convey the same facts, but the linked form is more fluid.

Could I replace 일이 많아서 with 해야 할 일이 많아서?

Yes. 해야 할 일이 많아서 (“because there were many tasks to do”) is more specific: it emphasizes “things I had to do.” It still uses the causal -아서 and works exactly the same way.

Is there any nuance difference between 와서/해서 and (이)어서 for cause?

-아서/어서 attaches to verb/adjective stems to mean “because,” and you choose 아 or 어 based on the final vowel of the stem. There’s no 와서 for adjectives; 하다 verbs take 해서 (e.g., 공부를 많이 해서). (이)어서 is not a separate pattern—adjectives like 많다 just use 많아서.