Learn Hardcore Korean: seonmureun modu siktak wie isseoyo. - All the gifts are on the dining table.

은 is the topic particle. It tells the listener “we’re talking about the gifts” and sets 선물 as the theme of the sentence. By contrast, 이/가 marks the grammatical subject more neutrally.

Using 선물은 emphasizes “as for the gifts…”
If you said 선물이 모두 식탁 위에 있어요, you’re simply stating “the gifts are on the table” without that topical “speaking of…” nuance.

Is 모두 acting as an adjective or an adverb, and what exactly does it modify?

Here, 모두 is an adverb meaning all or all of them. It modifies the implied phrase “are on the table,” telling us that every single gift is located there. You could think of it like “all together” or “all of them” in English.

If you wanted to use it directly with a noun you’d say 모두의 선물, where 모두의 becomes a possessive adjective (“everyone’s gifts” or “all of the gifts”).

Why is there an 에 attached to 위, making 위에?

위 by itself means “top” or “above.” To mark it as a static location (“on top of”), Korean needs the locative particle 에. So:

위 = top/above (noun)
위에 = on top of (location phrase)

This 에 tells us “where” the gifts are staying.

Could I say 선물은 식탁 위에 모두 있어요 instead? Would the meaning change?

Yes, that word order is also perfectly natural. The difference is in emphasis:

선물은 모두 식탁 위에 있어요 puts slight focus on 모두 (“all of them…”).
선물은 식탁 위에 모두 있어요 flows as “as for the gifts, they’re on the table—all of them.”

In practice, both convey the same basic idea, and native speakers often place 모두 before or after the location phrase interchangeably.

What’s the difference between 모두 and 다? Could I say 선물은 다 식탁 위에 있어요?

Both 모두 and 다 mean “all,” but they have subtle style differences:

모두 is a bit more formal or literary.
다 is more colloquial and common in everyday speech.

So 선물은 다 식탁 위에 있어요 is perfectly fine and might even sound more natural in casual conversation.

Why do we use 있어요 here? Could we use a different form like 있습니다 or the plain 있어?

있어요 is the polite (non-formal) ending, used in everyday speech with strangers, coworkers, or anyone you’re not extremely close to. Alternatives:

있습니다 is the formal ending (used in presentations, formal announcements).
있어 is the informal/casual ending (used with friends or younger people).

All three share the same meaning (“there are…/it is located…”), just differing in politeness level.