bunkasai no mise de yakisoba wo urukoto ni narimasita.

In this sentence, 店 most naturally means a stall, booth, or stand at a school cultural festival, not a permanent store.

So 文化祭の店で means something like:

• at the booth/stall at the cultural festival
• in our cultural festival stand

At school festivals in Japan, classes or clubs often run temporary food stands, games, or exhibits, so 店 is commonly used in that context.

The particle の links two nouns and shows a relationship between them.

Here:

• 文化祭 = cultural festival
• 店 = stall / booth / shop

So 文化祭の店 means:

• the cultural festival’s stall
• a stall at the cultural festival

In natural English, we usually do not translate の as of directly. We just make it sound natural.

で marks the place where an action happens.

The action here is 売る = to sell, so 店で tells you the location of that action:

• 店で売る = sell at the stall

Compare:

• で = where an action happens
• に = destination, existence, time point, etc. in other uses

So 文化祭の店で焼きそばを売る means to sell yakisoba at the festival stall.

こと can turn a verb phrase into a noun-like idea.

Here:

• 焼きそばを売る = to sell yakisoba
• 焼きそばを売ること = the act of selling yakisoba / selling yakisoba as a matter

This is necessary because the pattern 〜ことになる uses こと.

So the sentence structure is:

• 焼きそばを売ることになりました
• It has been decided that we/I will sell yakisoba
• literally: It became the case that [we/I] sell yakisoba

〜ことになりました means it has been decided that..., it turned out that..., or I/we ended up ... depending on context.

So:

• 焼きそばを売ることになりました = It has been decided that I/we will sell yakisoba = I/we ended up selling yakisoba = I/we are going to be selling yakisoba

A very important nuance is that 〜ことになる often sounds like the result of circumstances, a plan, or a decision made by a group, not purely the speaker’s own voluntary choice.

This is one of the most common questions, and the difference is important.

• 〜ことにしました = I decided to do...
• 〜ことになりました = It has been decided that... / It turned out that...

So:

• 焼きそばを売ることにしました = I decided to sell yakisoba

• 焼きそばを売ることになりました = It was decided that I/we would sell yakisoba = maybe by the class, club, teacher, committee, or the situation

In your sentence, なりました suggests the speaker is talking about a decision that has already been made, often not presented as purely their own personal decision.

Because the decision happened already.

The sentence is not saying the selling is already finished. It is saying that the situation became decided.

So the timeline is:

1. Before: it was undecided
2. Now: the decision has been made
3. Later: they will sell yakisoba

That is why なりました is natural.

English does something similar:

• We are going to sell yakisoba
• It was decided that we would sell yakisoba

The action is future, but the decision is already complete.

The subject is omitted, which is very normal in Japanese.

Depending on context, it could mean:

• I ended up selling yakisoba
• We ended up selling yakisoba
• Our class/club will sell yakisoba

Japanese often leaves out subjects when they are understood from context. In a school festival context, we or our class/club is often the most natural interpretation.

を marks the direct object of the verb.

Here:

• 焼きそば = yakisoba
• 売る = to sell

So 焼きそばを売る means to sell yakisoba.

This is a very standard pattern:

• パンを食べる = eat bread
• 本を読む = read a book
• 野菜を売る = sell vegetables

Yes, that sentence is possible, but the nuance is different.

• 文化祭の店で焼きそばを売ります = I/we will sell yakisoba at the festival stall

This simply states the plan or future action.

• 文化祭の店で焼きそばを売ることになりました = It has been decided that I/we will sell yakisoba at the festival stall

This emphasizes that a decision or arrangement has been made.

So 〜ことになりました adds the feeling of a decided outcome, not just a future action.

Yes, it is politely phrased.

The key polite part is:

• なりました instead of plain なった

So:

• 焼きそばを売ることになった。 = casual
• 焼きそばを売ることになりました。 = polite

This is a very natural polite sentence for everyday conversation, class discussion, or speaking to someone you are not very casual with.

Yes. Depending on context, a very natural English feeling might be:

• We’ve been put in charge of selling yakisoba at the cultural festival booth.
• It was decided that we’d sell yakisoba at our cultural festival stand.
• We ended up selling yakisoba at the cultural festival booth.

These translations capture the nuance of 〜ことになりました better than a word-for-word translation.

So the sentence is not just about the action sell. It is also about the fact that this became the plan or assignment.