i aebeun waipaiga eobseumyeon jal an dwaeyo.

은/는 marks the topic of the sentence, and 이/가 often marks the subject of a clause.

So in:

이 앱은 와이파이가 없으면 잘 안 돼요.

• 이 앱은 = as for this app
• 와이파이가 없으면 = if there is no Wi-Fi

The main topic is this app, so it gets 은.
Inside the conditional part, Wi-Fi is the thing whose existence is being talked about, so it gets 가.

A natural way to feel the structure is:

• As for this app, if Wi-Fi isn’t available, it doesn’t work well.

없으면 comes from:

• 없다 = to not exist / to not have
• -으면 = if / when

So:

• 없으면 = if there isn’t any / if you don’t have it

In this sentence, 와이파이가 없으면 means:

• if there is no Wi-Fi
• or more naturally, if you don’t have Wi-Fi

Because Korean often expresses having something through existence, 와이파이가 없다 literally means Wi-Fi does not exist, but in everyday English we usually translate it as there’s no Wi-Fi or you don’t have Wi-Fi.

Yes, 잘 usually means well, but in negative sentences it often means properly, successfully, or smoothly.

So:

• 잘 돼요 = it works well
• 잘 안 돼요 = it doesn’t work well / it doesn’t work properly

Here, 잘 안 돼요 does not mean it well doesn’t work word-for-word. It is a very common expression meaning something like:

• it doesn’t function properly
• it doesn’t work very well
• it doesn’t go smoothly

Yes. It comes from 되다, which is a very common verb with many meanings, including:

• to become
• to be possible
• to work / function
• to be okay / acceptable

In this sentence, 되다 means to work / function.

So:

• 돼요 = it works / it functions
• 안 돼요 = it doesn’t work / it doesn’t function

That means:

• 잘 안 돼요 = it doesn’t work well

This use of 되다 is extremely common with machines, apps, websites, phones, and plans.

Examples:

• 인터넷이 안 돼요. = The internet isn’t working.
• 프린터가 안 돼요. = The printer isn’t working.
• 로그인이 안 돼요. = Login isn’t working.

Because the verb in this sentence is 되다, not 하다.

• 하다 = to do
• 되다 = to work / become / be possible, depending on context

An app is not usually described as doing in this pattern. Instead, Korean commonly says whether something works or doesn’t work, and that is often expressed with 되다.

So:

• 잘 안 돼요 = it doesn’t work well
• 잘 안 해요 would not fit this meaning

Not exactly. 와이파이 is the subject of the conditional clause, not of the whole sentence.

Structure:

• 이 앱은 = topic of the whole sentence
• 와이파이가 없으면 = conditional clause: if there is no Wi-Fi
• 잘 안 돼요 = main clause: doesn’t work well

So the full sentence is organized like this:

As for this app, if Wi-Fi is absent, it doesn’t work well.

This is why both 은 and 가 can appear in the same sentence without any problem.

In casual speech, people sometimes drop particles, so you may hear something like:

• 이 앱은 와이파이 없으면 잘 안 돼요.

However, the full and clearer version is:

• 이 앱은 와이파이가 없으면 잘 안 돼요.

For learners, it is better to keep 가 here, because it shows the grammar clearly and sounds more standard.

돼요 is the polite informal style, also called 해요체. It is very common in everyday conversation.

So this sentence is polite and natural in many situations:

• talking to someone you don’t know well
• talking to a customer
• speaking generally politely

Other styles:

• 이 앱은 와이파이가 없으면 잘 안 돼. = casual
• 이 앱은 와이파이가 없으면 잘 안 됩니다. = more formal
• 이 앱은 와이파이가 없으면 잘 안 돼요. = polite everyday style

Using 은 emphasizes this app as the topic: as for this app.

If you said 이 앱이, it would sound more like you are specifically identifying this app as the subject in contrast to others, depending on context.

Compare:

• 이 앱은 와이파이가 없으면 잘 안 돼요.
  As for this app, it doesn’t work well without Wi-Fi.

• 이 앱이 와이파이가 없으면 잘 안 돼요.
  Grammatically possible in some contexts, but less natural as a neutral statement. It can sound more marked or contrastive.

In simple explanations, 은/는 often sounds more natural when giving general information about something.

They are similar, but not exactly the same.

• 없으면 = if there is no... / when there is no...
• 없을 때 = when there is no... / at times when there is no...

In this sentence, 없으면 is better because it expresses a condition:

• If there’s no Wi-Fi, the app doesn’t work well.

If you use 없을 때, it sounds more like describing a situation or time:

• When there’s no Wi-Fi, this app doesn’t work well.

Both can work in many contexts, but -으면 is especially natural for if conditions.

Not necessarily. 잘 안 돼요 usually suggests:

• it doesn’t work well
• it doesn’t work properly
• it has trouble working

This can be weaker than a total failure.

If you want to say it absolutely does not work, you might say:

• 아예 안 돼요. = It doesn’t work at all.
• 전혀 안 돼요. = It doesn’t work at all.
• 작동하지 않아요. = It does not operate. (more formal)

So 잘 안 돼요 often implies poor performance, not always complete failure.

Yes. 와이파이 is the Korean loanword for Wi-Fi.

Its syllables are:

• 와
• 이
• 파
• 이

Korean often writes borrowed English words phonetically, so Wi-Fi becomes 와이파이.

You will very commonly see and hear this word in Korea.

Here are some natural variations:

• 이 앱은 와이파이가 없으면 제대로 작동하지 않아요.
  = This app doesn’t function properly without Wi-Fi.
  More formal / technical.

• 이 앱은 와이파이 없이는 잘 안 돼요.
  = This app doesn’t work well without Wi-Fi.
  Using 없이 = without.

• 이 앱은 와이파이가 없으면 사용하기 불편해요.
  = This app is inconvenient to use without Wi-Fi.
  Slightly different nuance.

The original sentence is very natural and conversational.