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. By saying 어제는, you’re essentially saying “as for yesterday…” or “yesterday (speaking of)….” It sets 어제 (yesterday) as the frame of reference for the whole sentence. If you used 어제가, it would simply mark 어제 as the subject (“yesterday did X”), but using -는 makes it clear you’re talking about that day’s context.

Why is it 일이 많아서 and not 일을 많아서?

The adjective 많다 (“to be many/much”) takes a subject (이/가), not an object (을/를). Grammatically, 일 is the subject of 많다: “there was a lot of work.” That’s why you use 일이 많아서—일 + 이. If it were a transitive verb like 하다, you’d see 을/를 (e.g., 일을 했다).

How does 많아서 work? Why not 많았어서?

많아서 is the connective (causal) form of 많다. You take the adjective stem 많- and add -아서/어서 to mean “because….” This form does not carry tense—tense is shown only in the final verb. So even though the reason (“there was a lot of work”) happened in the past, you still use 많아서, not 많았어서. The past tense appears on 바빴어요.

What tense and speech level is 바빴어요?

바쁠 is the stem of 바쁘다 (“to be busy”). You add the past-tense marker -았- (which becomes 바빴-) and then the polite ending -어요. So 바빴어요 is past tense, polite/formal speech (the familiar 요-form you’d use with strangers or acquaintances.

What’s the role of 정말, and can I use something else?

정말 is an adverb meaning “really” or “truly,” used here to intensify 바빴어요. You could substitute 진짜, 아주, or 매우 to tweak the nuance. (Be careful with 너무—it can imply “too much.”) Placement is flexible, but adverbs of degree normally go right before the adjective: 정말 바빴어요.

What’s the difference between using -아서 and -니까 for “because”?

Both -아서/어서 and -니까 link clauses with a causal meaning, but:
• -아서/어서 is neutral and just states a background reason (“A happened, so B happened”). It’s the most common connector in everyday speech.
• -니까 is more explicit about “because” and can carry a stronger or more formal tone; it’s also used when asking permission or giving instructions (“since…, please…”).
In this sentence, 일이 많아서 simply lays out the reason matter-of-factly. You could say 일이 많았으니까 정말 바빴어요, but that shifts nuance toward “precisely because there was so much work, I was really busy.”