sseuregibongtuga da tteoreojyeoseo pyeonuijeomeseo keun geollo du jang sasseoyo.

That is a very common question.

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

• 쓰레기봉투가 떨어지다 = the trash bags run out
• 다 = all, completely
• -아서/어서 = because, so

So 다 떨어져서 means because they were completely used up / because we ran out.

Because 떨어지다 is an intransitive verb here.

The trash bags are not the object of an action like use or buy. They are the thing that ran out, so Korean marks them as the subject with 가.

• 쓰레기봉투가 다 떨어졌어요 = the trash bags ran out
• not 쓰레기봉투를 다 떨어졌어요

So 가 is natural because the sentence is describing the state of the trash bags.

Because 큰 is the form used before a noun.

크다 is the dictionary form, meaning to be big.
When it describes a following noun, it changes to 큰.

• 크다 = to be big
• 큰 봉투 = a big bag
• 큰 거 = a big one

So in this sentence, 큰 is the normal modifier form.

거 is the casual spoken form of 것, which means thing.

So:

• 큰 것 = a big thing / a big one
• 큰 거 = a big one

In this sentence, 거 stands in for the omitted noun, which is understood from context as 쓰레기봉투.

So 큰 거 means something like the big one or the larger kind.

Here, -로 shows the chosen option or selection.

In shopping or choosing contexts, A로 often means:

• as A
• in A type
• I’ll take A

So 큰 걸로 means the big one / the larger size, please or I chose the large kind.

It does not literally mean movement to something here.

A very common pattern is:

• 작은 걸로 주세요 = Please give me the small one
• 이걸로 할게요 = I’ll go with this one

Because Korean often leaves out nouns that are already obvious from context.

Once 쓰레기봉투 has already been mentioned, saying 큰 거 is enough to mean:

• the big trash bags
• the larger size

English often repeats nouns more than Korean does. Korean is very comfortable with saying just 거 or leaving the noun understood.

장 is the counter for flat, thin items.

It is used for things like:

• paper
• tickets
• cards
• sheets
• bags like this

Trash bags are treated as flat bag pieces, so 두 장 is natural.

So:

• 한 장, 두 장, 세 장...

is the right counting pattern here.

Because Korean usually uses native Korean numbers with most everyday counters.

So with 장, you normally say:

• 한 장
• 두 장
• 세 장
• 네 장

rather than Sino-Korean numbers like 일, 이, 삼.

So 두 장 is the normal everyday form for two sheets/bags.

-에서 marks the place where an action happens.

Since the action buying happened at the convenience store, Korean uses 편의점에서.

• 편의점에서 샀어요 = I bought it at the convenience store

This is different from -에, which is often used for destinations, locations of existence, or time.

Here, because buying is an action happening there, -에서 is the correct choice.

In everyday Korean, the object marker 를/을 is often omitted when the meaning is already clear.

The full idea is still bought two large trash bags, but Korean does not always need to mark the object explicitly in casual speech.

A fuller version could be something like:

• 큰 것으로 두 장을 샀어요
• 쓰레기봉투를 큰 것으로 두 장 샀어요

But in natural conversation, 큰 걸로 두 장 샀어요 sounds very normal.

Korean usually puts almost everything before the verb.

So this sentence is structured like:

• 편의점에서 = at the convenience store
• 큰 걸로 = the large kind / large ones
• 두 장 = two bags
• 샀어요 = bought

So the order is basically:

[where] + [what kind] + [how many] + [verb]

This is a very common Korean sentence pattern.

The dictionary form is 사다, meaning to buy.

In the past polite form, it becomes 샀어요.

This happens through contraction:

• 사다
• 사 + 았어요
• 샀어요

So 샀어요 simply means bought in polite speech.

This is partly a cultural point.

In Korea, trash bags are often official city garbage bags that people buy at places like:

• convenience stores
• supermarkets
• neighborhood shops

So 편의점에서 쓰레기봉투를 샀어요 sounds very normal in a Korean context.

A native English speaker might expect trash bags to come from a supermarket, but in Korea, a convenience store is a very ordinary place to buy them.