naeil bol jaryoneun chaeksang wie namgyeo duseyo.

볼 comes from the verb 보다 (to see / to look at / to read / to review, depending on context).

In this sentence, 볼 is the form that modifies a noun, so 볼 자료 means materials to look at tomorrow or materials that will be reviewed tomorrow.

So it is not a full verb by itself here. It is describing 자료.

• 보다 = to see / look at
• 볼 자료 = material(s) to see/look at/read

This is a very common question.

• 내일 볼 자료 = materials that will be looked at tomorrow
• 내일 보는 자료 = materials that one is looking at tomorrow / is in the process of looking at tomorrow

In this kind of sentence, Korean usually uses -ㄹ/을 before a noun when talking about something that is going to happen later relative to that noun.

So 볼 자료 sounds natural for materials for tomorrow’s review / materials to look at tomorrow.

By contrast, 보는 자료 usually sounds more like:

• material currently being looked at
• material habitually looked at
• ongoing action

So 볼 is the natural choice here.

는 is the topic particle.

So 자료는 means something like:

• as for the materials
• the materials,

The sentence is setting the materials for tomorrow as the topic, and then saying what should be done with them.

So the structure is roughly:

• 내일 볼 자료는 = as for the materials for tomorrow
• 책상 위에 남겨 두세요 = please leave them on the desk

It can also have a slight contrastive feeling, like:

• As for the materials for tomorrow, leave them on the desk
  (maybe other items should be handled differently)

It can be either.

Korean nouns usually do not have to show singular vs. plural as clearly as English does. So 자료 can mean:

• a document
• some material
• materials
• documents

The exact number depends on context.

If the speaker wanted to make plurality extra clear, they could say 자료들, but in many normal situations Korean simply uses 자료.

책상 위에 means on the desk or more literally on top of the desk.

Breakdown:

• 책상 = desk
• 위 = top / upper side
• 에 = location/destination marker

The particle 에 is used because this is the place where the materials are supposed to be left.

With verbs involving placement or resulting location, 에 is very common:

• 책상 위에 두세요 = put it on the desk
• 방에 놓으세요 = place it in the room

So 책상 위에 is the natural way to say on the desk here.

This is made of two parts:

• 남기다 = to leave behind
• 두다 = to put/keep something in a certain state

Together, 남겨 두다 means something like:

• leave it there
• leave it set aside
• leave it and keep it that way for later

So it often sounds more intentional than just 남기다 by itself.

In this sentence, the feeling is: Please leave the materials on the desk so they stay there for tomorrow.

Yes, 남기세요 could work, but 남겨 두세요 adds an extra nuance.

Compare them:

• 남기세요 = leave it
• 남겨 두세요 = leave it there for later / leave it set aside

The auxiliary verb 두다 often gives the sense of:

• doing something in advance
• leaving something in a certain state
• keeping it that way for future use

So in this sentence, 남겨 두세요 sounds very natural because the materials are being left there for tomorrow.

-세요 makes the sentence a polite request or polite instruction.

So 남겨 두세요 means:

• Please leave it
• Please leave them

It is softer and more polite than a plain command.

Compare:

• 남겨 둬 = leave it (casual)
• 남겨 두세요 = please leave it (polite)
• 남겨 두십시오 = please leave it (more formal)

So -세요 is a very common polite ending when asking someone to do something.

Not by grammar alone.

내일 볼 자료 literally just means materials to be looked at tomorrow. The sentence does not explicitly say who will look at them.

It could mean:

• the speaker will look at them tomorrow
• the listener will look at them tomorrow
• both will
• a group will

Korean often leaves subjects and pronouns unstated when they are understood from context.

So the exact person is usually clear from the situation, not from the sentence itself.

Yes.

The sentence does not explicitly say you, but the ending -세요 shows that the speaker is telling the listener to do something.

So the understood subject is basically:

• (you) please leave the materials on the desk

Omitting subjects like this is extremely common in Korean.

You may see both, but learners are usually safest writing 남겨 두세요.

That is because 두다 here is functioning like an auxiliary verb, and Korean textbooks usually teach learners to separate it:

• 남겨 두세요

In real life, attached forms like 남겨두세요 are also commonly seen.

So:

• 남겨 두세요 = safest textbook-style spacing
• 남겨두세요 = also seen in actual usage

If you are a learner, using 남겨 두세요 is a good habit.

Yes. Even if the meaning is already known, this is useful to understand.

A very literal breakdown is:

• As for the materials to look at tomorrow, please leave them on the desk.

More natural English translations might be:

• Please leave the materials for tomorrow on the desk.
• Please leave tomorrow’s materials on the desk.
• Please leave the materials we’ll review tomorrow on the desk.

Which one fits best depends on context, especially how 자료 is being used.