siroi kami wo itimai nooto ni hasande okimasu.

白い is an i-adjective, so it can directly modify a noun:

• 白い紙 = white paper

You do not need の or な here.

• 白の紙 can exist in some contexts, but it sounds more like paper of the color white and is less basic/plain than 白い紙
• 白な紙 is incorrect, because 白 is not a na-adjective in this use

So 白い紙 is the normal way to say white paper.

一枚 is a counter meaning one flat item.

Japanese often uses special counters depending on the shape/type of the object. 枚 is used for thin, flat things such as:

• paper
• shirts
• plates
• tickets

So:

• 紙を一枚 = one sheet of paper

Here, 一枚 tells you that the speaker means one sheet.

This is a very common Japanese pattern.

In Japanese, when a noun has a counter, the object marker often comes right after the noun, and the counter follows after that:

• 紙を一枚

This still means one sheet of paper as the direct object.

A useful way to think about it is:

• 紙を marks paper as the thing being acted on
• 一枚 adds the quantity: one sheet

This structure is very natural in Japanese.

Yes, both are possible, but the nuance and style are a little different.

• 白い紙を一枚
  very natural, conversational, and common

• 白い紙一枚を
  also natural; the quantity feels a bit more tightly connected to the noun phrase

• 一枚の白い紙を
  grammatically correct, but a little more formal or written-sounding

In everyday speech, 白い紙を一枚 is a very standard choice.

The particle に marks the place/target where the paper is being inserted.

• ノートに挟む = to insert/place something into a notebook, between its pages

Here に shows the location or destination of the action.

So the structure is roughly:

• white paper = thing being inserted
• notebook = place it is inserted into

That is why ノートに is used.

Not exactly.

挟む（はさむ） means to put something between two things, to sandwich, or to tuck in between.

In this sentence, it suggests putting the sheet between the pages of the notebook.

By contrast:

• 入れる = to put in / insert
• 挟む = to place between, tuck between

So 挟む gives a more specific image than 入れる. It is not just putting the paper into the notebook in a general sense, but slipping it between the pages.

Because this is the correct て-form of 挟む.

挟む is a -む verb, and verbs ending in -む / -ぶ / -ぬ change like this in the て-form:

• 読む → 読んで
• 遊ぶ → 遊んで
• 死ぬ → 死んで
• 挟む → 挟んで

So:

• 挟んでおきます = will put/tuck it in and leave it that way / will do it in advance

〜ておく is a very important grammar pattern. It often means one of these:

1. do something in advance, in preparation
2. do something and leave it that way

So 挟んでおきます can mean:

• I’ll tuck it into the notebook beforehand
• I’ll put it into the notebook and leave it there

In many cases, both ideas are present at the same time.

Compared with plain 挟みます, 挟んでおきます adds the feeling of preparation or leaving the result in place.

The sentence ends with ます, so it is in the polite non-past style.

• 挟んでおきます = polite
• 挟んでおく = plain/casual

The polite form does not change the basic meaning. It just changes the level of formality.

So this sentence is something you could say in normal polite conversation.

The subject is omitted, which is very normal in Japanese.

Depending on context, it could mean:

• I’ll put one sheet of white paper in the notebook
• We’ll put one sheet of white paper in the notebook
• even something like Please put one sheet..., depending on situation and intonation

Japanese often leaves out subjects when they are obvious from context. That is one of the biggest differences from English.

They are probably just there to help learners read the sentence more easily.

Normally, Japanese is written without spaces:

• 白い紙を一枚ノートに挟んでおきます。

In teaching materials, spaces are sometimes added to show word boundaries, but that is not standard everyday writing.