i oseun jipeoboda danchuga deo jal eoullyeo boyeoyo.

Because they are doing different jobs.

• 이 옷은 sets the overall topic: as for this item of clothing
• 단추가 marks the thing being described inside that topic: buttons are the thing that seems to suit it better

Korean often uses a topic + subject structure like this. So the sentence is not strange at all; it is very typical Korean.

보다 after a noun means than in comparisons.

So:

• 지퍼보다 = than a zipper / than zippers

A very common comparison pattern is:

• A보다 B가 더 ...
• B is more ... than A

So here, buttons are being compared with zippers.

Because Korean normally uses both.

• 보다 marks what you are comparing against
• 더 means more

So the standard pattern is:

• A보다 B가 더 좋다
• B is better than A

You can sometimes omit 더 if the comparison is already obvious, but keeping it sounds more complete and natural in most cases.

Because 잘 어울리다 is a very common expression meaning:

• to suit well
• to go well with
• to match nicely

Here 잘 does not mean skillfully. It means something like well / nicely.

So:

• 단추가 어울려요 = buttons suit it
• 단추가 잘 어울려요 = buttons suit it well / look especially fitting

-아/어 보이다 means to look / seem.

So:

• 어울려요 = it suits / it matches
• 어울려 보여요 = it looks like it suits / it seems to match

Using 보여요 makes the statement sound more like a visual impression. It is a little softer and more subjective, as if the speaker is judging by appearance.

So this sentence is not just stating a hard fact; it sounds like an opinion based on how the clothing looks.

Because 어울리다 does not usually treat the matching item as a direct object.

In Korean, the thing that suits or matches is often marked as the subject:

• 단추가 이 옷에 어울리다
• Buttons suit this clothing

So 단추가 is correct.

If you used 단추를, it would sound wrong with 어울리다 in this structure.

That idea is understood, even though it is not stated directly.

A fuller version could be:

• 이 옷에는 지퍼보다 단추가 더 잘 어울려 보여요

Here 이 옷에는 makes the relationship more explicit: on / with this clothing.

But in your sentence, 이 옷은 already sets the topic, so Korean naturally leaves the rest understood. This kind of omission is very common.

Korean nouns usually do not have to mark singular vs. plural unless it matters.

So:

• 단추 can mean a button or buttons
• 지퍼 can mean a zipper or zippers

In natural English, this sentence is usually expressed with buttons, because we are talking about a style choice in general. But the Korean itself does not force that number.

Yes. The dictionary form is 어울리다.

When it conjugates with -어, it becomes:

• 어울리어

That is then contracted to:

• 어울려

So:

• 어울리다 → 어울려요
• 어울리다 → 어울려 보이다 → 어울려 보여요

This is a normal contraction, not a special meaning change.

Yes, that is a very natural translation.

Korean 어울리다 expresses the idea of things matching or suiting each other, so English often adds words like:

• with
• on
• for

depending on what sounds natural.

A more literal translation is closer to:

• As for this garment, buttons seem to suit it better than a zipper does

But looks better with buttons than with a zipper is an excellent natural translation.

Korean 옷 can mean:

• clothes / clothing in general
• a garment / an item of clothing

So 이 옷은 can naturally mean:

• these clothes, in some contexts
• this clothing
• this item of clothing
• this garment

In this sentence, English often sounds best with this garment or this outfit, depending on context.

Yes, that also works.

Both are natural:

• 지퍼보다 단추가 더 잘 어울려 보여요
• 단추가 지퍼보다 더 잘 어울려 보여요

The first one follows the very common comparison pattern:

• A보다 B가 더 ...

That pattern makes the comparison especially clear. Changing the order mainly shifts the focus slightly, but the meaning stays the same.