madoguti de moratta hikae ha, nakusanai you ni tetyou ni hasande okimasu.

A natural way to break it up is:

• 窓口でもらった控えは = as for the copy/slip I received at the counter
• なくさないように = so that I don’t lose it
• 手帳に挟んでおきます = I’ll tuck it into my planner/notebook and leave it there / keep it there

So the sentence has:

1. a topic: 窓口でもらった控えは
2. a purpose/preventive expression: なくさないように
3. the main action: 手帳に挟んでおきます

Japanese often puts the item being talked about first, then explains why and what will be done with it.

窓口 literally means a service window or counter. In real-life Japanese, it often refers to:

• a reception desk
• a ticket window
• a bank counter
• an office counter where paperwork is handled

So here it means the place where the speaker received the 控え.

Here, で marks the place where the action happened.

So:

• 窓口で もらった = received at the counter

Even though もらう is to receive, Japanese commonly uses で for the location where that receiving took place.

Compare:

• 駅で会った = met at the station
• 店でもらった = received at the store
• 窓口でもらった = received at the counter

Because it describes 控え, and the receiving happened before the main action.

• 窓口でもらった控え = the copy/slip that I got at the counter

This is a noun-modifying clause. In English, we often use that I got.... In Japanese, the clause comes before the noun.

So もらった is past because the slip was already received earlier.

Yes. This is a very common Japanese pattern.

• 窓口でもらった控え

Literally, this is something like:

• the copy that was received at the counter
• more naturally, the copy I got at the counter

Japanese does not need a relative pronoun like that, which, or that I. The whole clause simply goes before the noun.

Other examples of the same pattern:

• 昨日買った本 = the book I bought yesterday
• 友だちにもらった手紙 = the letter I got from a friend

控え has several meanings, but here it means a copy, stub, receipt copy, or document kept for reference.

In office, banking, application, or administrative contexts, 控え often means:

• a duplicate
• a customer copy
• a slip you keep
• a record for later reference

So this is probably some paper the speaker should not lose.

It means so as not to lose it or to make sure I don’t lose it.

Breakdown:

• なくす = to lose something
• なくさない = not lose
• ように = so that / in order to / so as to

So:

• なくさないように = so that I won’t lose it

This expression is very common when talking about prevention or caution:

• 忘れないようにメモします = I’ll write a note 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 so that you don’t catch a cold

Because なくす and なくなる are different verbs.

• なくす = to lose something
  
  • transitive
  • someone loses an object
• なくなる = to disappear / to be lost / to run out
  • intransitive
  • the thing itself disappears

Here, the speaker is talking about their own action of accidentally losing the slip, so なくす is the right verb.

• 控えをなくす = lose the slip

If you used なくならない, it would sound more like so that it does not disappear / so that it does not get lost, which is grammatically possible in some contexts, but it is less direct for this situation.

Because Japanese often leaves out information that is already clear from context.

The topic is already 窓口でもらった控えは, so after that, it is understood that:

• なくさないように = so as not to lose it
• 手帳に挟んでおきます = I’ll tuck it into my planner/notebook

English usually needs it, but Japanese often omits it when the listener can easily tell what is meant.

挟む means to insert between, to sandwich, or to tuck between pages/items.

So:

• 手帳に挟む = to tuck it into a planner/notebook, usually between the pages

The particle に marks the place where the paper is being put.

This is not just putting it on the notebook; it suggests putting the slip between pages so it stays there.

Depending on context, 手帳 could mean:

• a planner
• a diary
• a notebook
• a pocket organizer

～ておく adds the idea of doing something:

• in advance
• for later convenience
• to leave it in that state

So:

• 挟みます = I put/tuck it in
• 挟んでおきます = I’ll tuck it in and leave it there so it’s ready / safe for later

In this sentence, ～ておきます fits very well because the speaker is taking a precaution.

It has the feeling of:

• I’ll keep it there so I don’t lose it
• I’ll put it there for safekeeping

Because the sentence is talking about what will be done with that particular slip.

• 窓口でもらった控えは... = As for the slip I got at the counter...

Using は makes 控え the topic of the sentence.

If you used を, the sentence structure would feel different, because を marks a direct object of the main verb. But here the speaker first introduces the slip as the thing under discussion, then explains the precaution being taken with it.

So は sounds very natural.

Because the speaker is expressing a present/future intention or a habitual action in polite style.

• 挟んでおきます can mean:
  • I’ll tuck it in
  • I keep it tucked in
  • I’m going to tuck it in

Japanese non-past often covers both present and future, depending on context.

So this could mean either:

• the speaker is about to do it now, or
• this is what they normally do in that situation

Both are possible unless the wider context makes one clearer.