cheobangjeoni eobseumyeon i yageun sal su eobsgeodeun.

Because 없다 works with the thing that does not exist or is not present as the subject, not the direct object.

So:

• 처방전이 없다 = There is no prescription / You don’t have a prescription
• not 처방전을 없다

That is why 처방전 takes 이/가 here.

In this sentence, 처방전이 없으면 means if there is no prescription or more naturally if you don’t have a prescription.

-으면 / -면 makes an if/when clause.

Here:

• 없다 = to not exist / to not have
• 없으면 = if there isn’t / if you don’t have

So:

• 처방전이 없으면 = if there is no prescription / if you don’t have a prescription

This pattern is very common:

• 시간이 있으면 가요. = If I have time, I’ll go.
• 비가 오면 안 가요. = If it rains, I won’t go.

This is a very common learner question.

The verb phrase is 살 수 없다 = cannot buy.
If you marked 약 as the direct object in the most neutral way, you could say:

• 이 약을 살 수 없다

That is grammatically fine.

But in your sentence, 이 약은 uses the topic particle 은/는. This adds a nuance like:

• as for this medicine
• this medicine, you can’t buy it
• when it comes to this medicine, you can’t buy it

So the sentence is not just stating an action; it is highlighting this medicine as the topic or contrast.

That can imply something like:

• other items might be buyable, but this medicine is not
• the speaker is specifically talking about this medicine

Literally, it means to not be able to buy.

Breakdown:

• 사다 = to buy
• 살 = the modifier form of 사다
• 수 = way, means, possibility
• 있다 / 없다 after -(으)ㄹ 수 = can / cannot

So:

• 살 수 있다 = can buy
• 살 수 없다 = cannot buy

This is one of the standard Korean patterns for expressing ability or possibility.

Both can mean cannot buy, but they are not always exactly the same in tone.

• 못 산다 often sounds like cannot in a more direct, practical sense
• 살 수 없다 can sound a bit more neutral or formal, like it is not possible to buy

In this sentence, 살 수 없거든 fits well because the speaker is explaining a rule or restriction:

• 처방전이 없으면 이 약은 살 수 없거든.
• You can’t buy this medicine without a prescription, you know / because...

It sounds a bit like it’s not allowed / it’s not possible under the rules.

-거든 adds an explanatory, background-giving nuance.

In this sentence, it feels like the speaker is giving a reason or reminding the listener of something the listener may not know:

• 처방전이 없으면 이 약은 살 수 없거든.
• You can’t buy this medicine without a prescription, you know.
• It’s because you can’t buy this medicine without a prescription.

It often has a tone like:

• just so you know
• you see
• the thing is
• because...

So this is not just a plain factual statement. It sounds more like the speaker is explaining the situation.

By itself, -거든 is not polite. It is casual or plain-style speech.

So:

• 없거든 = casual / plain
• 없거든요 = polite

A polite version would be:

• 처방전이 없으면 이 약은 살 수 없거든요.

That sounds natural in conversation when explaining something politely.

Korean often leaves out subjects when they are understood from context.

So 이 약은 살 수 없다 literally looks like:

• As for this medicine, cannot buy

But in natural English, we understand the missing subject as something like:

• you can’t buy this medicine
• people can’t buy this medicine

The exact subject depends on context, but Korean does not need to state it if it is obvious.

이 means this.

So:

• 이 약 = this medicine
• 그 약 = that medicine
• 저 약 = that medicine over there

In your sentence, 이 약은 means this medicine (as for this medicine).

Yes, 처방전 usually means a prescription, especially the written prescription issued by a doctor.

So in this sentence:

• 처방전이 없으면 = if you don’t have a prescription

Depending on context, English might translate it as:

• without a prescription
• if you don’t have a prescription from a doctor

A very literal breakdown would be:

• 처방전이 = as for a prescription / a prescription
• 없으면 = if there is not / if you do not have
• 이 약은 = as for this medicine
• 살 수 없거든 = cannot buy, you see / because you can’t buy it

So a near-literal translation is:

• If there is no prescription, this medicine cannot be bought, you know.

But natural English would be:

• You can’t buy this medicine without a prescription.
• You need a prescription to buy this medicine.

Yes. In casual speech, particles are often dropped.

So you may hear:

• 처방전 없으면 이 약은 살 수 없거든.

This is very natural in conversation.

But the full form with 이 is clearer and more textbook-like:

• 처방전이 없으면 이 약은 살 수 없거든.

Both are natural, depending on style.

It would be used when someone is explaining a restriction about medicine.

For example:

• Someone wants to buy a certain drug at a pharmacy
• A friend asks why they can’t just buy it
• A pharmacist or friend explains the rule

The tone with -거든 suggests the speaker is giving an explanation, such as:

• You can’t just buy this one, because it requires a prescription.
• This medicine isn’t available without a prescription, you know.

So it sounds explanatory rather than just coldly factual.