mensetu ni hituyouna syorui wo kaban ni iremasita.

A natural reading is:

めんせつ に ひつような しょるい を かばん に いれました。

Word readings:

• 面接 = めんせつ
• 必要 = ひつよう
• 書類 = しょるい
• 入れました = いれました

Also, かばん is often written in kana, but you may also see カバン or the kanji 鞄.

They are doing two different jobs.

• 面接に必要な: here に means for in the pattern N に 必要だ / 必要な N, meaning necessary for N
• かばんに入れました: here に marks the destination/place something is put into

So:

• 面接に必要な書類 = documents necessary for the interview
• かばんに入れました = put them into the bag

Same particle, different functions.

Because 必要 normally uses the pattern N に 必要 to mean necessary for N.

So:

• 面接に必要な書類 = documents necessary for the interview

Using の here would sound unnatural.
の connects nouns in a different way, like possession or category.

For example:

• 面接の書類 could mean interview documents or documents related to the interview
• 面接に必要な書類 specifically means documents that are needed for the interview

So に is the correct particle with 必要 in this structure.

必要 is a na-adjective.

When a na-adjective comes directly before a noun, it takes な:

• 必要な書類 = necessary documents
• 静かな部屋 = a quiet room

When it is at the end of a sentence, it does not use な:

• 書類は必要です。 = The documents are necessary.

So in this sentence, 必要 is modifying 書類, which is why it becomes 必要な書類.

It is one noun phrase built from left to right:

• 面接に = for the interview
• 必要な = necessary
• 書類 = documents

So the whole chunk 面接に必要な書類 means:

the documents that are necessary for the interview

In Japanese, modifiers come before the noun they describe.
So instead of saying documents necessary for the interview, Japanese says interview-for necessary documents.

を marks the direct object of the verb.

In this sentence:

• 書類を = the documents are the thing being put into the bag

So:

• 書類を入れました = put in the documents

A very literal breakdown would be:

• [documents] + object marker + [put in]

This is the difference between a transitive and an intransitive verb.

• 入れる = to put something in (transitive)
• 入る = to enter / to go in / to be inside (intransitive)

Here, the speaker is actively putting the documents into the bag, so Japanese uses 入れる:

• 書類をかばんに入れました = I put the documents into the bag.

If you used 入りました, the meaning would change:

• 書類がかばんに入りました = The documents went into the bag / fit in the bag.

So 入れました is correct because someone is causing the action.

Japanese often omits the subject when it is obvious from context.

So even though English usually says I put the documents into the bag, Japanese can simply say:

面接に必要な書類をかばんに入れました。

The listener will usually understand that the subject is:

• I
• we
• or whoever is being talked about in the conversation

This omission is very normal in Japanese.

入れました is:

• past tense
• polite form

Base verb:

• 入れる = to put in

Polite non-past:

• 入れます = put / will put

Polite past:

• 入れました = put

Casual past:

• 入れた

So this sentence is speaking politely about a completed action.

It can be either, depending on context.

Japanese nouns usually do not change form for singular vs. plural:

• 書類 can mean document
• or documents

In this sentence, English will often translate it as documents, because that sounds natural, but Japanese itself does not force that distinction.

The same is true for many Japanese nouns.

Some parts can move, but not everything.

Because the particles show each word’s role, Japanese word order is somewhat flexible. For example, these are both natural:

• 面接に必要な書類をかばんに入れました。
• かばんに面接に必要な書類を入れました。

Both mean the same basic thing.

However, the modifier must stay attached to the noun:

• 面接に必要な書類 must stay together as one unit

So you cannot break it up in unnatural ways.

A useful rule is:

• whole marked chunks can sometimes move
• but adjective/noun combinations should stay together