bunkasai de mise wo tetudaukoto ni narimasita.

こと turns the verb 手伝う (to help) into a noun-like phrase.

So:

• 手伝う = to help
• 手伝うこと = helping / the act of helping / to help

This nominalized phrase can then be used before に なりました.

In this sentence, 手伝うことになりました means something like it has been decided that I will help or I ended up helping.

Because the sentence is not simply saying I helped. It is saying that it was decided that I will help, or I came to be in a situation where I will help.

Compare:

• 店を手伝いました。 = I helped at the shop.
• 店を手伝うことになりました。 = It has been decided that I will help at the shop.

So ことになる is about a decision, arrangement, or resulting situation, not the completed action itself.

ことになる often means:

• it has been decided that...
• it turned out that...
• I am now supposed to...
• I ended up...

It often sounds less direct and less self-decided than ことにする.

So:

• 手伝うことにしました = I decided to help.
• 手伝うことになりました = It was decided that I would help / it turned out that I would help.

This can imply that the decision came from circumstances, a group decision, or someone else’s arrangement.

In the expression ことになる, the に is part of the grammar pattern.

The pattern is:

• dictionary form + ことになる

Examples:

• 日本へ行くことになりました。 = It has been decided that I will go to Japan.
• 毎日勉強することになっています。 = It is arranged/rule-based that I study every day.

You do not normally replace this に with another particle in this expression.

で marks the place or occasion where something happens.

Here, 文化祭で means:

• at the cultural festival
• during the school festival

So the festival is the setting in which the helping will happen.

Compare:

• 学校で勉強する = study at school
• 文化祭で店を手伝う = help with a shop at the cultural festival

This can feel a little strange to English speakers because in English you usually help a person more naturally than help a shop.

In Japanese, 手伝う can take を and mark the thing or activity being helped with:

• 仕事を手伝う = help with the work
• 家業を手伝う = help with the family business
• 店を手伝う = help with the shop / help run the stall

Here 店 probably means a festival stall/booth, not a permanent store in the ordinary sense.

So 店を手伝う means help out with the booth/shop operation.

Probably not. In the context of a 文化祭, 店 often refers to a temporary stall, booth, or class-run stand.

At Japanese school festivals, students often run things like:

• food stalls
• game booths
• themed cafes

So here 店 is best understood as the class’s stall/booth.

Great question. なりました is past tense because the change/decision has already happened.

So the sentence means:

• the decision has now been made
• the arrangement is now in place

Even if the actual helping will happen later, the becoming/decision happened already.

Compare the logic in English:

• It was decided that I would help.

The helping may still be in the future, but the deciding already happened.

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

Depending on context, it is probably:

• I
• we
• sometimes another person, if already clear from context

So the full idea might be:

• I ended up being assigned to help at a stall during the cultural festival.
• We ended up helping with a stall at the cultural festival.

Japanese often leaves out subjects when they are obvious from context.

Yes, very possibly.

Because it uses ことになりました instead of ことにしました, it can suggest:

• someone else decided
• the group decided
• circumstances led to it
• the speaker is reporting the arrangement rather than emphasizing personal choice

It does not always mean the speaker was unhappy about it, but it often sounds more neutral and less personally intentional.

Yes, and that is a very helpful way to read it:

• 文化祭で = at the cultural festival
• 店を手伝う = help with the stall/shop
• ことに = into the matter of / the situation of
• なりました = became

Natural chunked interpretation:

• At the cultural festival, it has been decided that I will help with a stall.

Seeing the sentence in chunks often makes the grammar much easier to understand.

They are similar, but the nuance is a little different.

• 文化祭で店を手伝う = help with a shop/stall at the cultural festival
• 文化祭の店を手伝う = help with the cultural festival’s shop/stall

The original with で focuses on the event as the location/setting. That is the most natural phrasing here.

Yes. It ends with なりました, which is the polite past form of なる.

Compare:

• casual: 文化祭で店を手伝うことになった。
• polite: 文化祭で店を手伝うことになりました。

So this sentence is appropriate in normal polite conversation, such as speaking to a teacher, classmate politely, or someone not very close.

A natural grammar-based paraphrase would be:

• It’s been decided that I’ll help at a stall during the cultural festival.

Other natural translations depending on context:

• I ended up helping with a stall at the cultural festival.
• I’m going to help run a stall at the cultural festival.
• I’ve been assigned to help with a stall at the cultural festival.

The exact English wording changes depending on how strongly you want to show the nuance of ことになりました.