jihacheoreseoneun inteones jeopsogi jal an dwaeyo.

Because each part does a different job.

• 지하철 = subway
• -에서 = in / at / on, marking the place where something happens
• -는 = topic marker

So 지하철에서는 means something like:

• in the subway, as for that situation...
• when it comes to being on the subway...

The -는 adds contrast or topical emphasis. It can imply something like:

• On the subway, internet access doesn’t work well
  maybe compared with other places where it does work fine.

Yes, absolutely.

• 지하철에서 is a plain location statement: in/on the subway
• 지하철에서는 adds a topic/contrast feeling

So:

• 지하철에서 인터넷 접속이 잘 안 돼요 = Internet access doesn’t work well on the subway.
• 지하철에서는 인터넷 접속이 잘 안 돼요 = On the subway, internet access doesn’t work well.

The version with -는 often sounds a little more like the speaker is setting the subway apart from other places.

인터넷 접속 means internet connection/access.

• 인터넷 = internet
• 접속 = connection, access, logging on

In this sentence, 인터넷 접속이 잘 안 돼요 means that getting connected to the internet does not work well.

It is a little more formal or technical than just talking about 인터넷 by itself. A speaker could also say things like:

• 인터넷이 잘 안 돼요 = The internet isn’t working well.
• 인터넷 접속이 잘 안 돼요 = Internet access/connection isn’t working well.

The sentence you gave is slightly more precise.

Here, 인터넷 접속이 is the thing being described as not working well.

So grammatically:

• 인터넷 접속이 = internet access/connection + subject marker
• 잘 안 돼요 = doesn’t work well / doesn’t go well / isn’t functioning properly

In English, we might not think of internet access as the subject in exactly the same way, but in Korean it is natural to mark it with 이/가 when saying whether it works or not.

Very literally, it is something like:

• 잘 = well
• 안 = not
• 돼요 = works / becomes possible / functions

So 잘 안 돼요 literally feels like it doesn’t work well or it doesn’t go well.

This is a very common Korean pattern. 되다 is often used when something functions, is possible, or works out.

Examples:

• 와이파이가 안 돼요. = The Wi-Fi doesn’t work.
• 로그인이 안 돼요. = Login doesn’t work / I can’t log in.
• 결제가 안 돼요. = Payment isn’t going through.

Because 되다 is extremely flexible in Korean. It can mean things like:

• to become
• to work
• to be possible
• to go through
• to be done

So 접속이 되다 means connection works / connection is possible / one can connect.

This is a very natural Korean way to say it. English often uses a specific verb like work, but Korean often uses 되다 in these situations.

Because 안 is a separate negation word.

Correct:

• 안 돼요

Not standard:

• 안되요

The base verb is 되다. When conjugated politely, it becomes 돼요. Then you put the negation word 안 before it:

• 안 + 돼요 → 안 돼요

This spacing is very important in Korean spelling.

Yes. It is present tense polite style.

• 되다 = to work / to become / to be possible
• 돼요 = present polite form
• 안 돼요 = does not work / is not possible

In context, it often has a general present meaning:

• Internet access doesn’t work well on the subway.

It can also describe the current situation:

• Right now, internet access isn’t working well on the subway.

Korean present tense often covers both general truths and current states.

돼요 is the standard polite style, often called 해요체.

It is appropriate in most everyday situations:

• with strangers
• with coworkers in casual-polite settings
• in daily conversation

Related forms:

• 안 돼 = casual
• 안 돼요 = polite
• 안 됩니다 = more formal

So your sentence is polite and natural for normal conversation.

Usually it means it doesn’t work well, not necessarily it never works at all.

That is the important role of 잘 here.

• 안 돼요 = it doesn’t work / it’s not possible
• 잘 안 돼요 = it doesn’t work well / it often fails / it’s unreliable

So the sentence suggests weak or unreliable internet access on the subway, rather than a complete impossibility in every single case.

Yes, Korean often leaves out subjects when they are obvious or unnecessary.

This sentence does not explicitly mention:

• I
• my phone
• the signal
• the subway internet

Instead, it simply states the situation:

• Internet access doesn’t work well on the subway.

Depending on context, English might expand it as:

• I can’t get good internet access on the subway.
• Internet access on the subway is poor.
• The internet doesn’t work well on the subway.

Korean does not need to state the experiencer if the meaning is already clear.

Most naturally, it means when you’re on the subway or in the subway environment.

That could include:

• inside the train
• underground sections
• the subway setting generally

In real conversation, people often use this kind of phrase a little broadly. The main idea is that while using the subway, internet access is bad.

So it is not usually interpreted in an overly strict way unless the context requires precision.

Yes. Very often, 에서는 gets contracted to 에선 in casual speech and writing.

So you may hear or see:

• 지하철에선 인터넷 접속이 잘 안 돼요.

This means the same thing as 지하철에서는. The full form is slightly more careful; the shortened form is very common and natural.