kyuukyuubako no naka ni houtai to kusuri ga haitte imasu.

の links two nouns and often works like of or 's in English.

• 救急箱 = first-aid kit / medicine box
• 中 = inside

So 救急箱の中 means the inside of the first-aid kit or more naturally inside the first-aid kit.

This is a very common pattern:

• 箱の中 = inside the box
• 学校の前 = in front of the school
• 机の上 = on the desk

Here, に marks the location where something exists.

So in 救急箱の中に, the に tells you the place where the bandages and medicine are.

This is a very common pattern with existence verbs:

• 机の上に本があります。 = There is a book on the desk.
• 部屋に猫がいます。 = There is a cat in the room.

In this sentence, the things exist / are located inside the first-aid kit, so に is the natural particle.

が marks the thing that exists or is in a certain state.

Because the sentence is basically saying Bandages and medicine are inside the first-aid kit, 包帯と薬 is the thing being described as existing there, so が is used.

A simple way to think about it:

• に = where
• が = what is there

So:

• 救急箱の中に = in the first-aid kit
• 包帯と薬が = bandages and medicine
• 入っています = are in / are inside

If you used は, the sentence would sound more like you are setting bandages and medicine up as the topic for contrast or further discussion.

Here, と means and when connecting nouns.

So:

• 包帯と薬 = bandages and medicine

This と is used when the list is complete or presented as a clear pair.

Compare:

• りんごとバナナ = apples and bananas
• 先生と学生 = teacher and student

A related particle is や, which means something more like such as or among other things. So if the sentence used 包帯や薬, it would suggest bandages and medicine are examples, not necessarily the full list.

入っています is the ている form of 入る.

With many verbs of movement or change, ている does not mean an action is happening right now. Instead, it often describes the resulting state.

So:

• 入る = to enter / go in
• 入っている = to be in; to have entered and now be inside

In this sentence, the meaning is not bandages and medicine are entering the first-aid kit right now. It means they are inside it.

This is similar to:

• ドアが閉まっています。 = The door is closed.
• 窓が開いています。 = The window is open.

Those are states resulting from a change.

This is a very common question.

• 入る is usually intransitive: something goes in / is in
• 入れる is usually transitive: someone puts something in

So:

• 薬が入っています。 = The medicine is inside.
• 薬を入れています。 = Someone is putting in / has put in the medicine.

In your sentence, the focus is on what is inside the first-aid kit, not on who put it there, so 入っています is the natural choice.

No. In this sentence, it describes a state, not an action in progress.

That is one of the trickier points of ている in Japanese. It can mean:

1. an ongoing action

   • 食べています = is eating

2. a resulting state

   • 結婚しています = is married
   • 座っています = is sitting
   • 入っています = is in / has entered and remains inside

So here, 包帯と薬が入っています means bandages and medicine are inside.

In 救急箱の中, 中 is read なか because it is being used as a noun meaning inside.

So:

• 中 = なか = inside

This is a very common reading in location expressions:

• 箱の中 = inside the box
• 家の中 = inside the house
• かばんの中 = inside the bag

Kanji often have more than one reading, and this is the normal one for this pattern.

The structure is:

place に thing が 入っています

More specifically:

• 救急箱の中に = in the first-aid kit
• 包帯と薬が = bandages and medicine
• 入っています = are inside

So the full pattern is roughly:

In [place], [thing] is/are in there.

This kind of word order is very normal in Japanese, where the location often comes before the thing that exists there.

Yes, sometimes, but the nuance is slightly different.

• 包帯と薬があります。 = There are bandages and medicine.
• 包帯と薬が入っています。 = Bandages and medicine are inside / are contained in it.

あります simply says something exists there. 入っています emphasizes that the items are contained inside something.

So in a sentence about the contents of a first-aid kit, 入っています feels especially natural.

The spaces are just for learners. Normal Japanese is usually written without spaces:

救急箱の中に包帯と薬が入っています。

Textbooks and beginner materials often add spaces to make the sentence easier to read and to show where words or particles begin and end.