Learn Hardcore Korean: jido-e giri pyosidwae isseoyo.

Because the verb is passive/resultative: 표시돼 있어요 (“is marked”). The thing that is marked is the subject, so 길이 is correct. If you used the active voice, you’d say: (사람들이) 지도를 만들 때 길을 표시해요 (“People mark roads when making a map.”).

What’s the difference between 표시돼 있어요, 표시돼요, and 표시됐어요?

표시돼 있어요: Focuses on the current resulting state: “it is (currently) marked.”
표시돼요: Present/habitual passive or an in-the-moment passive process: “it gets marked/it is being marked” (context-dependent).
표시됐어요: Simple past passive event: “it was marked” (at some point). It doesn’t inherently emphasize that the marking is still visible now, whereas …돼 있어요 does.

What exactly is 돼 in 표시돼 있어요? Is it the same as 되어?

Yes. 돼 is the standard contraction of 되어 (from 되다 + -어). Both 표시되어 있어요 and 표시돼 있어요 are correct; the contracted form is more common in speech. Avoid the incorrect spelling 되요; use 돼요 or the full 되어요.

How do you pronounce 돼?

It’s pronounced roughly “dwae” ([dwe]). So 표시돼 sounds like “pyo-si-dwae.” Don’t pronounce it like “de” or “doe.”

Why not say 표시해 있어요?

-어 있다 combines naturally with intransitives or passives to express a resulting state (e.g., 열려 있다, 붙어 있다, 표시돼 있다). Using it with an active/transitive like 표시하다 (“to mark”) is ungrammatical or very unnatural. Use the passive 표시되다 with -어 있다: 표시돼 있어요.

Can I use -에서 instead of -에 (i.e., 지도에서)?

Not here. -에서 marks the place where an action happens. 표시돼 있어요 describes a state, so -에 is the right choice. However, with an active verb it changes: 지도에서 길을 표시해요 (“On the map, (someone) marks roads”) is fine.

Does 길이 ever mean “length”? I’m worried about ambiguity.

Yes, 길이 can also be the noun “length.” Here, though, it’s clearly 길 + 이 (“road + subject marker”), meaning “road(s).” Context disambiguates. If you really want to avoid confusion, you can say 도로가 (“the road(s)”) or 도로가 지도에 표시돼 있어요.

Could I use 도로 instead of 길?

Yes. 도로 tends to sound more like “roads/roadways” (often official, drivable roads), while 길 is broader (“road, path, way”). On a map, both can work; choose based on nuance.

Is the word order fixed? Can I say 길이 지도에 표시돼 있어요?

Word order is flexible. Both are fine:

지도에 길이 표시돼 있어요 (sets the location first)
길이 지도에 표시돼 있어요 (puts the subject first) Use order to control what you’re foregrounding (location vs. subject).

What does adding -는 as in 지도에는 do?

-는/은 adds a topic/contrast. 지도에는 길이 표시돼 있어요 can imply “On the map (at least), the roads are marked” (perhaps contrasting with somewhere else where they aren’t).

How should I space this? Is 표시되어있어요 okay?

Standard spacing separates the auxiliary 있다:

Correct: 표시되어 있어요 / 표시돼 있어요
Nonstandard: 표시되어있어요, 표시 되 있어요, 표시 되있어요 Keep 돼/되어 attached to the main verb and 있어요 separate.

Is there a difference between 표시되다 and 표기되다?

표시되다: “to be marked/indicated” (lines, icons, colors, highlights). Natural for map symbols and routes.
표기되다: “to be written/notated” (orthographic representation, spellings, labels). On maps, use 표기되다 for names/labels being written, and 표시되다 for roads/routes/symbols.

How do I change the politeness/formality?

Casual: 표시돼 있어.
Polite (standard): 표시돼 있어요 / 표시되어 있어요.
Formal: 표시돼 있습니다 / 표시되어 있습니다. All mean the same; the difference is just register.