Learn Hardcore Korean: oneul eopmuga manhaseo bappayo. - I'm busy today because I have a lot of tasks.

The particle 가 is the subject marker, showing that 업무 (“work” or “tasks”) is what’s “many.” If you used 는, you’d be marking 업무 as the topic, which changes the nuance.

업무가 많아서 emphasizes “there is a lot of work, so…”
업무는 많아서 would sound odd here (it’d mean “as for work, since it’s a lot…” without a clear result clause).

What exactly does 업무 mean, and how is it different from 일?

업무 is a Sino-Korean word meaning “business tasks” or “official work,” often used in office or professional settings.
일 is more general—“work,” “thing to do,” or “matter.”
• If you’re talking about your daily chores or errands, you’d use 일.
• In a corporate email or meeting, 업무 sounds more formal and specific.

What role does the connector -아서 play in 많아서?

The suffix -아서/-어서 attaches to a verb or adjective stem to show cause (“because …”).
Here:
많다 (to be many) → stem 많- → 많아서 = “because (there’s) a lot (of work).”
It links that reason clause to the main clause (바쁘다).

Why do we attach -아서 to 많다 rather than to 많아요?

Connective endings like -아서 always attach to the dictionary form stem (drop -다), not to a conjugated form.
• 많다 → 많 + 아서 = 많아서
If you tried 많아요서, it wouldn’t follow Korean conjugation rules.

Why is there no 요 after 많아서, but only at the end in 바빠요?

In Korean, the politeness marker 요 goes on the final clause of the sentence.

많아서 is part of the reason clause, so it stays unmarked for politeness.
바빠요 is the main statement, so it carries the -요 to show polite speech.

Why is the subject “I” omitted in this sentence?

Korean often drops pronouns when the subject is clear from context. Here, it’s obvious the speaker means “I.”
Adding “I” (저는) is correct but optional.

How do you pronounce 많아서 in natural speech?

Because 많 ends with the final cluster ㄶ, when you add the vowel‐initial 아서, the ㅎ moves to the next syllable.
So 많아서 is pronounced [만하서] (man-ha-seo), not [만아서].

How would you say this sentence in a more casual (informal) style?

Drop the polite -요:
“오늘 업무가 많아서 바빠.”
You could also contract 업무 to 일, making it even more colloquial:
“오늘 일 많아서 바빠.”

Can I add 저는 at the beginning? What changes?

Yes. “저는 오늘 업무가 많아서 바빠요.”
Adding 저는 explicitly marks “I” as the topic and can:

Clarify who you’re talking about,
Emphasize that it’s your situation (useful if the context isn’t clear).
Otherwise, native speakers simply omit it when it’s obvious.