piryo eobsneun yeongsujeungeun beoryeoyo.

Literally, it is something like without need.

• 필요 = need, necessity
• 없다 = to not exist / to be absent
• 없는 = the noun-modifying form of 없다

So 필요 없는 영수증 is literally a receipt that has no need, which naturally becomes an unnecessary receipt or a receipt you don't need.

A fuller form would be 필요가 없는 영수증.

Here, 가 is often omitted because the phrase is so common and the meaning is clear. So:

• 필요가 없는 영수증 = fully explicit
• 필요 없는 영수증 = very natural, shorter everyday form

This kind of particle omission is common in Korean, especially in short modifying phrases.

Because Korean puts modifiers before the noun.

In English, you might say:

• a receipt that you don't need

In Korean, that descriptive part comes first:

• 필요 없는 영수증

So 없는 is modifying 영수증.

This is a very common learner question.

• 영수증을 버려요 = directly marks receipt as the object
• 영수증은 버려요 = makes receipt the topic: as for receipts..., throw them away

Using 은 here gives a nuance like:

• As for receipts you don't need, throw them away
• It can sound like a general rule or instruction
• It can also imply contrast: maybe needed receipts are kept, but unneeded ones are thrown away

So 은 is not wrong here at all; it changes the focus.

It can be either, depending on context.

In Korean, the polite -아요/어요 form is not always strictly just a plain statement. In the right context, it can sound like:

• a neutral instruction
• a gentle command
• a general rule

So 버려요 could mean:

• I throw it away
• We throw it away
• Please throw it away
• You throw it away in an instructional sense

In a notice, guide, or everyday instruction, it often feels like Please throw away unnecessary receipts.

The dictionary form is 버리다.

It changes like this:

• 버리다
• stem: 버리-
• • -어요
• 버리어요
• contracted to 버려요

So 버려요 is just the normal polite present form of 버리다.

In this sentence, yes, it means to throw away / discard.

But 버리다 is also a very common verb in Korean with a broader sense of getting rid of something. In other contexts, it can also appear as an auxiliary verb after another verb to add a meaning like do completely, sometimes with a nuance of regret or finality.

But here it is just the main verb: throw away.

It is omitted.

Korean often leaves out the subject when it is obvious from context. So the sentence does not explicitly say:

• you
• I
• we
• people in general

The listener is expected to understand it from the situation. In an instruction, the hidden subject is often effectively you.

Korean nouns usually do not need a plural marker when the meaning is already clear from context.

So 영수증 can mean:

• receipt
• receipts

depending on the situation.

Korean does have plural marking, like 영수증들, but it is often unnecessary and can sound less natural if the context already tells you that more than one receipt is meant.

Yes, you could.

Both are grammatical:

• 필요 없는 영수증은 버려요
• 필요하지 않은 영수증은 버려요

The difference is mostly in style and feel:

• 필요 없는 is shorter and very natural in everyday Korean
• 필요하지 않은 feels a bit more explicit and can sound slightly more formal or written

So the original sentence is a very natural everyday way to say it.