hyujiga da tteoreojyeoseo pyeonuijeomeseo hyujireul sayo.

휴지 can mean tissue paper in general, so in real life it may refer to tissues or toilet paper, depending on context.

In this sentence, English might naturally translate it as either:

• tissue
• toilet paper
• paper towels in some situations

Korean often leaves that detail to context unless it needs to be more specific.

Because 휴지 is the subject of 다 떨어지다 here.

The first clause is basically:

• 휴지가 다 떨어져서 = because the tissue/toilet paper ran out

So 가 marks the thing that has run out.

A simple way to think about it:

• 무엇이 다 떨어졌어요?
• 휴지가 다 떨어졌어요.

That is why 가 is natural here.

Here, 떨어지다 does not mean only to fall. With supplies or consumable items, it often means to run out or to be used up.

So:

• 떨어지다 = to run out / be used up
• 다 = completely, all
• 다 떨어지다 = to run out completely

Then -서 connects it to the next clause.

So 휴지가 다 떨어져서 means:

• because I’m all out of tissue/toilet paper
• because the tissue/toilet paper has run out

It comes from:

• dictionary form: 떨어지다
• connective ending: -어서

So the full combination is:

• 떨어지다 + -어서
• 떨어지어서
• contracted to 떨어져서

This kind of contraction is very common in Korean.

-아/어서 links two clauses. Very often it means:

• because
• so
• or a natural sequence of events

In this sentence, it gives the reason for the second action:

• 휴지가 다 떨어져서 편의점에서 휴지를 사요.
• Because I’m out of tissue/toilet paper, I buy some at the convenience store.

So here -서 is best understood as a reason/cause connector.

에서 marks the place where an action happens.

Here, the action is buying, and that action happens at the convenience store:

• 편의점에서 휴지를 사요. = I buy tissue at the convenience store.

This is different from 에, which is often used for:

• destination: 편의점에 가요 = I go to the convenience store
• location of existence: 편의점에 사람이 있어요 = There is a person at the convenience store

So:

• 편의점에 가요 = go to the convenience store
• 편의점에서 사요 = buy at the convenience store

Korean often repeats the noun where English would use it or some.

So this is very natural:

• 휴지가 다 떨어져서 편의점에서 휴지를 사요.

In English, repeating tissue might sound unnecessary, but in Korean it is clear and normal.

Could you omit it? Yes, if the context is already obvious:

• 휴지가 다 떨어져서 편의점에서 사요.

But by itself, repeating 휴지를 makes the sentence clearer.

사요 is the -아요/-어요 polite style form of 사다.

• dictionary form: 사다 = to buy
• polite present: 사요 = buy / am buying

About the other forms:

• 삽니다 is also polite, but more formal
• 샀어요 is past tense = bought
• 살 거예요 means will buy / am going to buy

So 사요 is a normal everyday polite form.

Also, Korean present tense can be a little broader than English. Depending on context, 사요 can sound like:

• I buy
• I’m buying
• I buy some now
• sometimes even a near-future action

By itself, it usually sounds like a specific situation:

• the tissue ran out
• so the speaker buys some at the convenience store

However, because Korean present tense is flexible, it could also describe a repeated or typical action if the context supports that.

If you want to make the past situation clearer, you could say:

• 휴지가 다 떨어져서 편의점에서 휴지를 샀어요.
  = Because I ran out of tissue, I bought some at the convenience store.

If you want a future intention, you could say:

• 휴지가 다 떨어져서 편의점에서 휴지를 살 거예요.
  = Because I’m out of tissue, I’m going to buy some at the convenience store.

Yes. It uses the everyday polite -요 style, often called 해요체.

• 사요 = polite
• full sentence sounds natural in everyday conversation

Other possible styles would be:

• 휴지가 다 떨어져서 편의점에서 휴지 사.
  casual, less polite
• 휴지가 다 떨어져서 편의점에서 휴지를 삽니다.
  more formal

So the original sentence is polite but not stiff.