keiyakusyo no hikae ha nakusanai you ni fairu ni hasande okimasu.

It means a copy of the contract.

• 契約書 = contract document
• 控え = copy, duplicate, one kept for reference

In business Japanese, 控え often means the copy you keep on hand, not just any copy in the abstract.

の links the two nouns and shows the relationship between them.

So:

• 契約書 = contract
• 控え = copy
• 契約書の控え = the contract’s copy / a copy of the contract

This is a very common noun-modifying pattern in Japanese.

は marks 契約書の控え as the topic of the sentence.

So the sentence is talking about the copy of the contract, and then saying what will be done with it.

There can also be a slight feeling of as for the contract copy... depending on context, but here it mainly just introduces the topic naturally.

なくさない is the negative form of なくす, which means to lose something.

So:

• なくす = to lose
• なくさない = not lose

Here it means so that it won’t get lost or more literally so as not to lose it.

Because なくす and なくなる are different verbs.

• なくす = to lose something
• なくなる = to disappear / to be gone

In this sentence, the idea is taking care not to lose the copy, so なくさない is the natural choice.

If you used なくならない, the nuance would be more like so that it does not disappear / go missing, which is possible in some contexts but less direct here.

〜ないように means so that not..., in order not to..., or to make sure not to....

So:

• なくさないように = so as not to lose it / so that it doesn’t get lost

This pattern is often used for precautions, intentions, or preventive actions.

Examples:

• 忘れないようにメモします。 = I’ll write a memo so I don’t forget.
• 風邪をひかないように気をつけてください。 = Please be careful not to catch a cold.

Because 〜ように is a set grammar pattern.

In this structure:

• Vないように + action
• so that ... not happen, do action

So in this sentence:

• なくさないように
• so as not to lose it

The に is part of the grammar, not a separate location marker.

挟む literally means to insert between, to sandwich, or to put something between other things.

With ファイル, it suggests putting the document into the file/folder, especially by slipping it between pages or into a holder.

So ファイルに挟んで gives a more specific image than just putting it somewhere.

Both could work, but the nuance is different.

• 入れる = to put in
• 挟む = to insert between things

So:

• ファイルに入れる = put it in the file
• ファイルに挟む = slip it into the file / place it between sheets or in a folder sleeve

挟む sounds a bit more physical and specific.

〜ておく means to do something in advance, to do something ahead of time, or sometimes to leave something as it is for later.

Here, 挟んでおきます means:

• I’ll put it in the file in advance
• I’ll file it away so it’s taken care of

This adds the nuance that the action is being done as a precaution or for future convenience.

Here に marks the place/destination where the document is being put.

So:

• ファイルに挟む = insert it into the file

This is similar to other verbs of placement:

• 箱に入れる = put it in a box
• 机の上に置く = place it on the desk

It is saying I’ll file it so it won’t get lost.

The main action is:

• ファイルに挟んでおきます = I’ll put it in the file / I’ll file it away

And the reason/purpose is:

• なくさないように = so that it won’t get lost

So the whole sentence is about taking a preventive action.

Yes:

• 契約書の控えは = as for the copy of the contract
• なくさないように = so that it won’t get lost / so as not to lose it
• ファイルに = into the file
• 挟んでおきます = I’ll insert it / file it away in advance

A natural English-style gloss would be:

As for the copy of the contract, I’ll put it in a file so it doesn’t get lost.

The verb can be written as 失くす, but in everyday Japanese it is very often written in hiragana as なくす.

That is common for many verbs, especially when:

• the kanji is not essential for clarity
• the writer wants a softer, simpler look
• the sentence is practical rather than literary

So なくさない is completely natural.