kikai no naibu wo kakuninsita.

の connects two nouns and shows that the second noun belongs to, relates to, or is part of the first.

So:

• 機械 = machine
• 内部 = interior / inside
• 機械の内部 = the inside of the machine / the machine's interior

This is a very common pattern in Japanese:

• 車のドア = the car's door
• 家の中 = the inside of the house
• 学校の先生 = the school's teacher / teacher at the school

In this sentence, 機械の内部 is one noun phrase.

内部 means interior, inside, or internal part.

It sounds a bit more formal or technical than everyday words like 中.

Compare:

• 中（なか） = inside, interior, middle
  
  • more common in everyday speech
• 内部（ないぶ） = internal area, interior, inside
  
  • more formal, technical, or precise

So 機械の内部 sounds natural for talking about a machine, device, system, or structure.

を marks the direct object of the verb.

The verb here is 確認した (confirmed / checked / inspected), and what was checked is 機械の内部.

So:

• 機械の内部を = the inside of the machine (as the thing being checked)
• 確認した = checked / confirmed

Even though English often says checked inside the machine, Japanese treats the inside of the machine as the thing directly checked, so を is natural.

確認した is the past plain form of 確認する.

Breakdown:

• 確認 = confirmation / checking
• する = to do
• 確認する = to confirm / to check
• 確認した = confirmed / checked

This is a very common type of verb in Japanese: a noun + する.

Other examples:

• 勉強する = to study
• 連絡する = to contact
• 予約する = to reserve
• 確認する = to confirm/check

した is the plain past form, while しました is the polite past form.

So:

• 確認した。 = checked / confirmed.
  
  • plain style
• 確認しました。 = checked / confirmed.
  
  • polite style

The sentence you were given is in plain style, which is common in:

• written notes
• reports
• casual narration
• textbooks
• dictionary-style example sentences

It is not rude by itself; it just has a less formal tone than しました.

Japanese often leaves out the subject when it is clear from context.

So 機械の内部を確認した。 could mean:

• I checked the inside of the machine.
• We checked the inside of the machine.
• He/She/They checked the inside of the machine.

Japanese does this very often. If the speaker, topic, or actor is already understood, it does not need to be stated.

If you wanted to include the subject, you could say:

• 私が機械の内部を確認した。 = I checked the inside of the machine.
• 彼が機械の内部を確認した。 = He checked the inside of the machine.

But in many situations, leaving it out sounds more natural.

It is helpful to think of the sentence in chunks:

• 機械の内部 = the inside of the machine
• 機械の内部を = the inside of the machine + object marker
• 機械の内部を確認した = checked the inside of the machine

So yes, 機械の内部 is one noun phrase, and then を確認した tells you what happened to it.

This is a very useful reading strategy in Japanese: find the noun phrase first, then attach the particle and verb.

They can all be translated as something like check depending on context, but the nuance is different.

• 見る = to see / to look at
  
  • very broad and general
• 調べる = to investigate / examine / look into
  
  • often implies researching or examining in detail
• 点検する = to inspect / do a safety or maintenance check
  
  • often used for equipment, machinery, systems
• 確認する = to confirm / verify / check
  
  • often means making sure something is correct, present, or in the expected condition

So 機械の内部を確認した suggests something like:

• checked the inside of the machine
• confirmed the condition of the machine's interior
• made sure everything inside the machine was okay

If the sentence were 機械の内部を点検した, it would sound more like a formal inspection.

Yes, you could say 機械の中を確認した, and it would still be understandable.

However, there is a nuance difference:

• 機械の中 = inside the machine
  
  • more everyday and direct
• 機械の内部 = the machine's interior / internal area
  
  • more formal, technical, or report-like

For machinery, manuals, technical explanations, or formal descriptions, 内部 often sounds more natural.

The spaces are almost certainly there for learners.

Normal Japanese writing would usually be:

機械の内部を確認した。

Japanese does not normally put spaces between words the way English does. Educational materials sometimes add spaces to make the structure easier to see.

By itself, 機械 does not clearly mark singular vs. plural.

So depending on context, it could refer to:

• the machine
• a machine
• sometimes even machines, though singular is more likely here

Japanese usually relies on context rather than articles like a or the.

So in a real situation, the listener would know whether it means:

• a particular machine already being discussed
• a machine in general
• one machine in a work report

The basic order is:

[object] を [verb]

So here:

• 機械の内部 = object
• を = object marker
• 確認した = verb

Japanese verbs usually come at the end of the sentence.

A very literal word-for-word order would be:

• machine's interior + object marker + checked

Natural English becomes:

• I checked the inside of the machine.
• The inside of the machine was checked.
  depending on context

Yes, it is natural and clear.

It sounds especially natural in contexts like:

• maintenance reports
• repair work
• technical writing
• inspection logs
• narration of a procedure

For example, in a work report, 機械の内部を確認した。 sounds very normal: concise, factual, and practical.