mise ga simaru mae ni, pan wo kaimasu

Because が marks the subject of the smaller clause 店が閉まる (the shop closes).

In this sentence, 店が閉まる前に means before the shop closes. Inside that clause, 店 is specifically the thing doing the action 閉まる.

Using は would shift the nuance and make 店 more of a topic, which is less natural here unless you are contrasting it with something else.

So:

• 店が閉まる前に = before the shop closes
• 店は閉まる前に... would sound like you are setting the shop up as a topic in a different way

This is a very common pattern in Japanese.

When a verb comes before a noun or before expressions like 前に, it usually appears in the plain form, not the polite form.

Here, 閉まる前に literally works like:

• 閉まる = closes / will close
• 前 = before
• に = at / before

So the structure is:

• verb (plain form) + 前に

Examples:

• 寝る前に = before sleeping / before I go to bed
• 食べる前に = before eating
• 店が閉まる前に = before the shop closes

Even though the whole sentence ends politely with 買います, the verb inside this kind of clause stays plain.

Because Japanese structures this idea differently.

Japanese says something more like:

• the shop closes + before

So 店が閉まる前に is literally at the time before the shop closes.

A useful way to think of it is that 前 is a noun meaning before/front, and the clause before it describes what that before is relative to.

So:

• 店が閉まる前に = before the shop closes
• literally: at the before of the shop closing

This is normal Japanese word order.

前に means before in a time sense here.

In this sentence:

• 店が閉まる前に = before the shop closes

Breaking it down:

• 前 = before
• に = marks the point in time

So 前に often follows a verb or noun to mean before doing X or before X happens.

Examples:

• 日本へ行く前に = before going to Japan
• 食事の前に = before the meal
• 寝る前に = before sleeping

The に marks a time point.

In 店が閉まる前に、パンを買います, it shows the action 買います happens at a point before the shop closes.

You can think of it as linking the main action to that time frame.

This is why 前に is such a common set phrase:

• 出かける前に = before going out
• 勉強する前に = before studying

So while learners often memorize 前に as one chunk meaning before, it helps to know that に is doing real grammatical work.

Because パン is the direct object of 買います.

The verb 買う / 買います means to buy, so the thing being bought takes を.

• パンを買います = buy bread

This is the standard object marker in Japanese.

More examples:

• 本を読みます = read a book
• 水を飲みます = drink water
• 靴を買います = buy shoes

It is the non-past form, which can mean either present or future depending on context.

Here, it most naturally means a future or intended action:

• I will buy bread before the shop closes

Japanese does not force the same present/future distinction that English does in this form.

So 買います can mean:

• buy
• will buy
• sometimes usually buy, depending on context

In this sentence, because of before the shop closes, it clearly refers to a future action or planned action.

Because Japanese often uses plain form inside subordinate clauses, even when the main sentence is polite.

So this is perfectly normal:

• 店が閉まる前に、パンを買います。

The main sentence ends politely with 買います, but the embedded clause uses plain 閉まる.

This happens in many patterns:

• 日本に行く人 = the person who goes to Japan
• 雨が降ったら、帰ります = if it rains, I will go home
• 時間があるとき、勉強します = when I have time, I study

This mix is not strange at all in Japanese grammar.

閉まる is intransitive: to close.

That means the shop closes by itself as the subject of the sentence:

• 店が閉まる = the shop closes

閉める is transitive: to close something.

Examples:

• 店が閉まる = the shop closes
• 店を閉める = someone closes the shop

In your sentence, the meaning is that the shop will close, so 閉まる is the correct choice.

Because Japanese often leaves out subjects when they are understood from context.

So:

• 店が閉まる前に、パンを買います

naturally means something like:

• I will buy bread before the shop closes

But Japanese does not need to say 私が or 私は unless there is a reason to emphasize or clarify it.

This omission is extremely common and natural.

Yes, お店 is very common and often sounds a little softer or more everyday in conversation.

So you could say:

• お店が閉まる前に、パンを買います。

Both are correct.

Very roughly:

• 店 = store/shop
• お店 = shop/store, with the polite prefix お

In many daily situations, お店 sounds very natural.

The comma is there to separate the time clause from the main action, making the sentence easier to read.

So:

• 店が閉まる前に、パンを買います。

The comma is helpful, but punctuation in Japanese is often more flexible than in English. You may sometimes see sentences like this without a comma, especially in shorter or more casual writing.

It does not change the grammar.

Yes, Japanese word order is somewhat flexible, especially with time expressions and other modifiers.

You could also say:

• パンを、店が閉まる前に買います。
• 店が閉まる前にパンを買います。

These all mean basically the same thing.

However, 店が閉まる前に、パンを買います is very natural because the time phrase comes first and sets the scene before the main action.

Yes. This is a useful grammar point.

前 is a noun meaning front or before. In this sentence it is being used in a time expression.

That is why a plain-form clause can come before it:

• 店が閉まる describes 前
• so 店が閉まる前 = the time before the shop closes

This is similar to how Japanese often places descriptive clauses before nouns.

So grammatically, 閉まる前 is not a random verb-plus-particle combination. It is actually a clause modifying the noun 前.