daizina fairu ha forudagoto pasokon ni hozonsite okimasu.

Because 大事 is a na-adjective in Japanese, not an i-adjective.

• Before a noun, a na-adjective takes な
• So:
  • 大事なファイル = important file(s)
  • not 大事いファイル

Compare:

• 静か → 静かな部屋
• 便利 → 便利なアプリ
• 大事 → 大事なファイル

は marks ファイル as the topic of the sentence.

So 大事なファイルは means something like:

• As for important files...
• Regarding important files...

Even though ファイル is the thing being saved, Japanese often uses は to set it up as the topic instead of using を directly.

A fuller, more explicitly object-marked version could be imagined as something like:

• 大事なファイルは、フォルダごとパソコンに保存しておきます。

Here, the listener understands that the important files are what will be saved.

Because は has replaced it as the topic marker.

In Japanese, when a noun is both:

• the thing you want to talk about, and
• the object of the verb,

it is very common for は to appear instead of を.

So the sentence is not wrong or incomplete. It is just focusing on important files as the topic:

• ファイルを保存します = save the files
• ファイルは保存します = as for the files, save them / the files, I’ll save them

This often adds a slight sense of contrast too, depending on context, such as at least the important files.

ごと means as a whole unit, together with everything in it, or sometimes each / by each unit, depending on context.

So フォルダごと most naturally means:

• as entire folders
• together with the folders
• folder-by-folder

In this sentence, it suggests that the files are not being saved one by one individually, but with their folders as complete units.

A similar pattern:

• 家ごと = the whole house
• 箱ごと = together with the box / the whole box
• 部署ごと = by department / department by department

Because ごと attaches directly to the noun and the whole expression works adverbially.

So:

• フォルダ + ごと → フォルダごと

This already means something like in whole-folder units or together with the folder, so no extra particle is required here.

That is normal for expressions with ごと.

Here に marks the destination/result location of the saving.

• パソコンに保存する = save onto the computer
• クラウドに保存する = save to the cloud

By contrast, で usually marks:

• the place where an action happens, or
• the means/tool used

So パソコンで保存する would sound more like save using the computer rather than save onto the computer.

In this sentence, the computer is the place the files end up, so に is the natural choice.

〜ておく adds the meaning of doing something in advance, for future convenience, or leaving it done/prepared.

So:

• 保存します = save
• 保存しておきます = save it in advance / save it and keep it that way for later

In this sentence, 保存しておきます suggests something like:

• I’ll save them ahead of time
• I’ll make sure they’re saved
• I’ll save them and leave them stored for future use

This is a very common grammar pattern:

• 買っておきます = buy it in advance
• メモしておきます = make a note of it beforehand
• 閉めておきます = close it and leave it closed

It is the polite non-past form of 置く（おく）, used in the grammar pattern 〜ておく.

So the structure is:

• 保存して = te-form of 保存する
• おきます = polite form of おく

Together:

• 保存しておきます

The non-past form in Japanese can mean:

• future: I will save it
• habitual: I save it
• intention/plan, depending on context

Here it most naturally means future or planned action.

The subject is omitted, which is extremely common in Japanese.

Depending on context, it could mean:

• I will save...
• We will save...
• You should save...
• They save...

Japanese often leaves out subjects when they are already understood from the situation.

So this sentence is perfectly natural even without 私が or another subject.

Not completely. Japanese word order is more flexible than English because particles show the grammatical roles.

This sentence is:

• 大事なファイルは フォルダごと パソコンに 保存しておきます。

But small rearrangements are possible, for example:

• 大事なファイルはパソコンにフォルダごと保存しておきます。

Both are understandable. The original order sounds natural and clearly presents:

1. the topic: important files
2. the manner/unit: as folders / folder-by-folder
3. the destination: onto the computer
4. the action: save in advance

Japanese often places the main verb at the end, but the phrases before it can move around somewhat.

It can mean either, because Japanese nouns usually do not mark singular vs. plural.

So ファイル could mean:

• a file
• files

In this sentence, context makes files more likely, especially because:

• 大事なファイル often refers to important files in general
• フォルダごと also suggests grouped content

But grammatically, Japanese itself does not force a singular/plural distinction here.