mauseureul chaeksang wie dueosseoyo.

Because 마우스 ends in a vowel sound in Korean.

In Korean, the object particle is:

• -을 after a consonant
• -를 after a vowel

Even though the English word mouse ends with an s sound, the Korean loanword is written 마우스, and the last syllable 스 has no final consonant 받침. So it counts as ending in a vowel, which is why you use 를:

• 마우스 + 를 = 마우스를

책상 위에 means on top of the desk or onto the top of the desk.

It breaks down like this:

• 책상 = desk
• 위 = top, upper side, surface
• 에 = location/destination particle

So 책상 위에 is literally at the top of the desk → naturally on the desk.

Korean often uses words like 위 (top), 아래 (bottom), 앞 (front), 뒤 (back) where English might just use a preposition like on, under, in front of, or behind.

Because 에 marks the place where something ends up or exists, while 에서 usually marks the place where an action happens.

In this sentence, the important idea is the location where the mouse was placed:

• 책상 위에 두었어요 = placed it on the desk

With 두다, the desk is the destination/resulting location, so 에 is the natural choice.

Compare:

• 책상 위에 두었어요 = I put it on the desk.
• 책상 위에서 would sound like the action is happening on top of the desk, which is not what this sentence is trying to say.

두었어요 comes from the verb 두다, which means to put, to place, or sometimes to leave something somewhere.

Here is the basic formation:

• 두다 → verb stem 두-
• past/perfect-like form 두었-
• polite ending -어요

So:

• 두었어요 = put / placed / left

In everyday speech, this often contracts to 뒀어요, which is very common.

They are often similar, and in many everyday situations both can be translated as put.

But there is a slight difference in feeling:

• 두다 often suggests putting something somewhere and leaving it there
• 놓다 often focuses more on placing/setting something down

So in a sentence like this, both can work:

• 마우스를 책상 위에 두었어요
• 마우스를 책상 위에 놓았어요

Both are natural. But 두다 can more strongly suggest that the mouse was placed there and remains there.

Because Korean often leaves out the subject when it is already clear from context.

So this sentence could mean:

• I put the mouse on the desk
• He/She put the mouse on the desk
• We put the mouse on the desk

The subject is simply not stated because it is understood from the situation.

If you wanted to include it, you could say:

• 제가 마우스를 책상 위에 두었어요 = I put the mouse on the desk.

Because Korean normally puts the verb at the end of the sentence.

A very common Korean word order is:

• subject + object + place + verb
• or more generally, everything comes before the verb

So:

• 마우스를 = the mouse
• 책상 위에 = on the desk
• 두었어요 = put

This is completely normal in Korean.

You can move some parts around for emphasis, but the verb usually stays at the end:

• 마우스를 책상 위에 두었어요
• 책상 위에 마우스를 두었어요

Both are possible.

두었어요 is in the polite casual style, often called 해요체.

It is:

• polite
• natural in everyday conversation
• less formal than 두었습니다
• more polite than 두었어

So this is a very common form for speaking to:

• strangers
• coworkers
• teachers
• people you are not close friends with

It can imply both, depending on context.

Because 두다 often has the sense of putting something somewhere and leaving it there, the sentence can feel like:

• I put the mouse on the desk
• I left the mouse on the desk

The exact English translation depends on the situation. If the meaning has already been given to you as put, that is perfectly fine, but the Korean verb can carry that extra nuance of something remaining in that place.

In everyday conversation, 뒀어요 is very common and sounds natural.

So these are both correct:

• 마우스를 책상 위에 두었어요
• 마우스를 책상 위에 뒀어요

The second one is just the contracted spoken form. Learners should recognize both.

Yes, but the meaning becomes less specific.

• 책상 위에 = on top of the desk
• 책상에 = at/on/to the desk, depending on context

If you say:

• 마우스를 책상에 두었어요

people may still understand that you mean on the desk, because that is the most natural interpretation. But 책상 위에 is clearer and more exact because it specifically says on top of the desk.