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jihacheoreseoneun deiteoga jal an dwaeyo.

지하철에서는 데이터가 잘 안 돼요.jihacheoreseoneun deiteoga jal an dwaeyo.
The mobile data doesn’t work well on the subway.
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Questions & Answers about jihacheoreseoneun deiteoga jal an dwaeyo.

What does 지하철에서는 mean, and why is it -에서는 instead of just -에서?

지하철에서 means in/at the subway (on the subway) as the location where something happens.
Adding 는 → 지하철에서는 gives -에서는 a contrast/topic feel: as for in the subway / when it comes to the subway (but maybe elsewhere it’s fine).
So 지하철에서는 데이터가 잘 안 돼요 often implies it doesn’t work well in the subway (unlike other places).

Is 지하철 “subway” the place (the system) or the train (inside the car)?

지하철 can refer to the subway system in general, but in this sentence 지하철에서는 is commonly understood as while you’re on the subway / in the subway environment (often inside the train or underground areas). If you want to be very explicit about being inside the train car, you might say 지하철 안에서는 (inside the subway).

Why is 데이터가 marked with -가? Could it be 데이터는?

-가 marks 데이터 as the grammatical subject of 안 돼요 (doesn’t work): Data (signal) doesn’t work well.
You can say 데이터는, which makes data the topic/contrast:

  • 지하철에서는 데이터는 잘 안 돼요 = As for data, it doesn’t work well on the subway (maybe calls/texts do).
    Using -가 here is the neutral, common choice.
What exactly does 데이터 mean in Korean? Is it “data” in general?

In everyday Korean, 데이터 usually means mobile data / cellular data (LTE/5G data connection), not “data” in the abstract. So this sentence typically means mobile internet/data signal doesn’t work well on the subway.

What does 잘 안 돼요 literally mean, and why use 되다?

Literally: 잘 = well, 안 = not, 돼요 (from 되다) = works / becomes possible / functions.
So 잘 안 돼요 = it doesn’t work well / it doesn’t go through well / it doesn’t function properly.
Korean often uses 되다 for “work/function” in a broad sense, especially for connections, payments, logins, apps, etc.

What’s the difference between 안 돼요 and 못 돼요 here?
  • 안 돼요 = it doesn’t work / it’s not working (for whatever reason) (more general).
  • 못 돼요 = it can’t work / it’s impossible (because of inability/conditions).
    For signal issues, natives usually say 안 돼요 because it simply isn’t working (whether temporarily or due to environment). 못 돼요 is possible but less natural in this specific situation.
Why is 잘 included? Could I just say 데이터가 안 돼요?

Yes, you can say 지하철에서는 데이터가 안 돼요 = Data doesn’t work on the subway.
Adding 잘 softens/nuances it to not working well (weak/spotty/unstable). That matches real-life subway reception better than implying it never works at all.

Is 안 돼요 about permission (“you can’t”) or about function (“it doesn’t work”)?

안 돼요 can mean both depending on context:

  • Permission/prohibition: 여기서 담배 피우면 안 돼요 (You can’t smoke here)
  • Function/condition (this sentence): 데이터가 안 돼요 (My data isn’t working)
    Here, because the subject is 데이터 and the setting is the subway, it’s clearly the function meaning.
What politeness level is 돼요? What are other natural variations?

돼요 is polite informal (common in daily conversation). Variations:

  • Casual: 지하철에서는 데이터 잘 안 돼.
  • More formal polite: 지하철에서는 데이터가 잘 안 됩니다.
    All are natural; choose based on who you’re speaking to.
How would I say “The Wi‑Fi doesn’t work well on the subway” instead?

Replace 데이터 with 와이파이 (Wi‑Fi):

  • 지하철에서는 와이파이가 잘 안 돼요.
    Or if you mean the subway’s public Wi‑Fi specifically:
  • 지하철 와이파이가 잘 안 돼요. (context-dependent)