kare ha neko wo heya ni tozikometa.

In this sentence, は and を are particles, and particles sometimes have special pronunciations.

• は is pronounced wa when it marks the topic
• を is pronounced o in modern Japanese

So the sentence is read:

かれ は ねこ を へや に とじこめた

は marks 彼 as the topic of the sentence: as for him.

So 彼は gives a neutral topic-comment structure:

• As for him, he locked the cat in the room

If you used が, it would put more focus on he as the one who did it, often with a nuance like:

• He is the one who locked the cat in the room

In a simple standalone example, 彼は is very natural.

Because 猫 is the direct object of the verb 閉じ込めた.

The verb 閉じ込める means to shut in, to lock up, or to confine someone or something. The thing being shut in is marked with を.

So here:

• 猫を = the cat as the thing he locked in

Here, に marks the place that the cat ends up in.

With verbs like put, send, trap, or shut in, に often marks the destination or resulting location.

So:

• 部屋に閉じ込めた = shut it into the room or more natural English, locked it in the room

The important idea is not just where the action happened, but where the cat was confined.

で usually marks the place where an action happens.

But in this sentence, the key point is the room as the place the cat was confined in, not just the place where he performed the action. That is why に is used.

A rough contrast:

• 部屋に閉じ込めた = locked it in the room
• 部屋で would sound more like the action took place at/in the room, which is not the intended structure here

So に is the normal choice with 閉じ込める.

閉じ込めた is the plain past form of 閉じ込める.

閉じ込める means:

• to shut in
• to lock up
• to confine

It is stronger than just 閉めた, which only means closed or shut.

For example:

• ドアを閉めた = closed the door
• 猫を部屋に閉じ込めた = shut the cat in the room so it could not get out

So 閉じ込めた has the idea of trapping or confining something inside.

It is treated as one verb, but it is also helpful to see how it is built.

It combines:

• 閉じる = to close
• 込める = to put into, pack into, confine into

Together, 閉じ込める gives the sense of:

• close something in
• shut something inside
• confine

Many Japanese compound verbs work like this, where the second part adds a nuance such as into, completely, or thoroughly.

It is the plain past form.

• dictionary form: 閉じ込める
• plain past: 閉じ込めた
• polite past: 閉じ込めました

So this sentence is in casual or plain style, which is also the style used in dictionary examples and many textbook sentences.

Yes, to some extent. Japanese word order is more flexible than English because particles show each word’s role.

For example, these are both natural:

• 彼は猫を部屋に閉じ込めた
• 彼は部屋に猫を閉じ込めた

Both mean basically the same thing.

However:

• the verb usually comes at the end
• the topic often appears near the beginning
• changing word order can slightly change emphasis

So the order is flexible, but not completely random.

Not as often as English uses he.

In natural Japanese, if the subject is already clear from context, it is often omitted. So in real conversation, people might simply say:

• 猫を部屋に閉じ込めた

and the listener would understand who did it from context.

Using 彼 is not wrong, but it can sound a little more explicit than English learners expect. In textbook sentences, subjects like 彼 are often included to make the grammar easier to follow.

Japanese nouns usually do not show articles such as a or the, and they usually do not have a special plural form either.

So:

• 猫 can mean a cat, the cat, or sometimes cats
• 部屋 can mean a room or the room

The exact meaning comes from context.

In this sentence, learners usually understand 猫 as the cat or a cat, depending on the translation being given. Japanese leaves that unsaid unless it needs to be made explicit.