keisatu ha youki no naka ni doku wo mitukemasita.

A natural breakdown is:

• 警察は = as for the police / the police
• 容器の中に = in the container
• 毒を = poison (marked as the thing found)
• 見つけました = found

So the basic pattern is:

Topic + location + object + verb

Japanese often puts the verb at the end, so the sentence literally feels like:

As for the police, in the container, poison found.

は marks the topic of the sentence. It tells you what the sentence is about.

So 警察は means something like:

• As for the police, ...
• The police, ...

If you used が instead, it would put more focus on the police as the subject, as if answering who found it.

Compare:

• 警察は容器の中に毒を見つけました。
  = The police found poison in the container.
  (neutral/topic-based statement)

• 警察が容器の中に毒を見つけました。
  = It was the police who found poison in the container.
  (more emphasis on who did it)

容器 means container.
中 means inside or middle.

When you put them together with の:

• 容器の中 = the inside of the container / inside the container

The particle の connects nouns and often works like of in English.

So:

• 容器 = container
• 中 = inside
• 容器の中 = container’s inside / inside the container

Here, に marks the location where something exists or is found.

In this sentence:

• 容器の中に = in the container

With a verb like 見つける (to find), the place where something is found is often marked with に.

So the structure is:

• [place] に
• [thing] を
• 見つける

Example:

• 机の上に本を見つけた。 = I found a book on the desk.

Usually, に is the natural choice here.

That is because に marks the place where something is located or where it is discovered to exist.
With 見つける, Japanese often treats the location as the place in which the object was found.

So:

• 容器の中に毒を見つけました = natural

Using で would sound less standard here, because で usually marks the place where an action is carried out, like:

• 学校で勉強する = study at school
• 公園で遊ぶ = play in the park

For finding something in a place, に is the usual particle.

を marks the direct object of the verb — the thing that is directly affected by the action.

Here, the verb is 見つけました (found), and the thing found is 毒 (poison).

So:

• 毒を見つけました = found poison

Even though the poison is in the container, it is still the object of find, so it takes を.

The dictionary form is 見つける.

• 見つける = to find
• 見つけました = found / have found

This is the polite past form.

Formation:

• 見つける
• stem: 見つけ
• polite present: 見つけます
• polite past: 見つけました

In this sentence, it means found.

警察 usually means the police as an organization or body, not a single officer.

So in this sentence, 警察は is best understood as:

• the police
• the police authorities

If you wanted to clearly say a police officer, you would more often use:

• 警察官 = police officer

So:

• 警察 = police
• 警察官 = police officer

The standard readings are:

• 警察 = けいさつ
• 容器 = ようき
• 中 = なか
• 毒 = どく
• 見つけました = みつけました

So the full sentence is read:

けいさつ は ようき の なか に どく を みつけました。

No, normal Japanese writing usually does not use spaces between words.

The sentence would normally be written as:

警察は容器の中に毒を見つけました。

Spaces are often added in teaching materials to help beginners see the parts of the sentence more clearly.

Japanese word order is more flexible than English because particles show each word’s role.

This sentence could also be reordered in some ways, for example:

• 警察は毒を容器の中に見つけました。

This is still understandable, because:

• は marks the topic
• を marks the object
• に marks the location

However, 容器の中に毒を見つけました sounds very natural because it presents the place first and then the thing found there.

The main thing that usually stays fixed is that the verb comes at the end.

中 has several readings, and the correct one depends on the word or expression.

Here, 中 means the physical inside of something, so it is read なか:

• 容器の中 = ようきのなか = inside the container

The reading ちゅう is used in many compound words, such as:

• 中国 = ちゅうごく
• 一年中 = いちねんじゅう
• 授業中 = じゅぎょうちゅう

So in this sentence, because it means a literal inside space, なか is the correct reading.

容器 is a correct and useful word, but it is a bit more formal or general than some everyday alternatives.

It means container, receptacle, or vessel.

Depending on context, everyday speech might use more specific words like:

• 箱 = box
• 瓶 = bottle/jar
• 入れ物 = container, case, holder

So 容器 sounds a little more neutral or technical, which fits well in reports, news, or formal writing.