ibento ga aru to, hiroba ni oozei no hito ga atumarimasu.

It has two parts:

1. イベントがあると
   = when/if there is an event

2. 広場に大勢の人が集まります
   = many people gather in the plaza

So the sentence is a conditional pattern:

[X] と、 [Y]
= when/if X happens, Y happens

In this sentence, the idea is not just one specific event, but a typical or expected result.

No. Here と is not the particle meaning and.

It is the conditional と, which means something like:

• when
• if
• whenever

So イベントがあると means:

• when there is an event
• whenever there is an event

This conditional と is often used when the result is:

• natural
• automatic
• habitual
• generally true

That fits this sentence well: when an event happens, people gather.

Japanese uses different existence verbs depending on what exists.

• ある = for inanimate things, events, objects
• いる = for people and animals

An event is not a living thing, so Japanese uses ある:

• イベントがある = there is an event

Even though an event is not a physical object, it is still treated grammatically like a non-living thing.

In イベントがある, the thing that exists is marked by が.

So:

• イベントがある = an event exists / there is an event

This is very common with ある and いる.

You can think of が here as marking the thing being identified as existing.

A learner may wonder why it is not イベントはある. That is possible in some contexts, but it changes the nuance:

• イベントがある = simply states that there is an event
• イベントはある = puts event in contrast or as a topic, something like as for the event, it exists

For a plain statement, が is the natural choice.

に marks the place that people gather to.

With 集まる, the location is often marked by に because it is the destination or point of convergence.

• 広場に集まる = gather in / gather at the plaza

This is different from verbs where で marks the place where an action happens.

For example:

• 広場でイベントがある = the event takes place in the plaza
• 広場に人が集まる = people gather in the plaza

So in your sentence, に is natural because the focus is where the people end up gathering.

Because this sentence has two clauses, and each clause has its own subject-like element.

Clause 1:

• イベントがある
• イベント is marked by が

Clause 2:

• 大勢の人が集まります
• 大勢の人 is marked by が

So the two が particles are not competing with each other. They belong to different parts of the sentence.

This is very normal in Japanese.

大勢の人 means a large number of people, many people, or a crowd of people.

Breakdown:

• 大勢 = a large number of people
• の = links it to 人
• 人 = people

So literally it is something like a large-number group of people.

Compared with たくさんの人, 大勢の人 often sounds a little more specifically like a lot of people as a crowd. It is especially common when talking about many people gathering somewhere.

Also, 大勢 is mostly used for people, not things.

Japanese non-past forms cover several meanings, including:

• present
• future
• habitual action
• general truth

Here 集まります is not just simple present in the English sense. It expresses a general or habitual result:

• When there is an event, many people gather.
• Whenever there is an event, many people gather.

So the non-past form is exactly what Japanese normally uses here.

The ます form also makes the sentence polite.

This is a very common question.

• 集まる = intransitive = to gather, to assemble
• 集める = transitive = to collect, to gather something

In your sentence, the people themselves gather, so Japanese uses 集まる:

• 人が集まる = people gather

If someone actively gathers people, then you would use 集める:

• 人を集める = gather people / bring people together

So:

• 大勢の人が集まります = many people gather
• not 大勢の人を集めます, because that would mean someone is collecting or gathering the people

Sometimes yes, but the nuance changes.

• と = when/whenever X happens, Y naturally follows
• たら = if/when X happens, then Y; often more specific or situational
• ば = if X, then Y; often more logical or formal

Here と is a very good choice because it suggests a regular pattern:

• イベントがあると、広場に大勢の人が集まります。
• Whenever there is an event, lots of people gather in the plaza.

If you said イベントがあったら, it could sound more like a particular future or imagined case:

• If there is an event, then people will gather

So と fits the general, habitual feel best.

Yes. In many cases, Japanese と can be understood as either if or when, depending on context.

But the nuance here is closer to:

• when there is an event
• whenever there is an event

That is because the second clause sounds like an expected result, not a doubtful possibility.

So although if is not wrong, when or whenever often matches the Japanese feeling better in this sentence.

Japanese does not need は in every sentence.

This sentence works perfectly with が because it is simply describing a situation:

• an event occurs
• many people gather

If you added は, the nuance would change. For example:

• イベントがあると、広場には大勢の人が集まります。

This version is possible and natural if you want to emphasize the plaza as the place where people gather. The には gives a bit of contrast or focus, something like in the plaza, many people gather.

But the original version is simpler and more neutral.

The main readings are:

• イベント = ibento
• 広場 = ひろば (hiroba)
• 大勢 = おおぜい (oozei)
• 人 = ひと (hito)
• 集まります = あつまります (atsumarimasu)

So the full sentence is read:

イベントがあると、ひろばにおおぜいのひとがあつまります。

The basic meaning would stay the same even if some parts move around, because the particles show each word’s role.

For example, these are also understandable:

• イベントがあると、大勢の人が広場に集まります。
• 広場に、イベントがあると大勢の人が集まります。
  This one is less neutral and depends more on context.

The most natural order is usually the one in your sentence or this very common alternative:

• イベントがあると、広場に大勢の人が集まります。
• イベントがあると、大勢の人が広場に集まります。

Both are natural. The difference is mostly about what the speaker wants to emphasize a little more.