daizina fairu ha betu no foruda ni hozonsite oku hou ga ii.

Because 大事 is a na-adjective.

• Before a noun, a na-adjective takes な:
  • 大事なファイル = an important file
• At the end of a sentence, it uses だ / です:
  • このファイルは大事だ = This file is important.

So 大事な is the correct form when it directly modifies ファイル.

別の means another, different, or separate.

In this sentence:

• 別のフォルダ = a different folder / a separate folder

Here, 別 is being used before a noun, and の connects it to フォルダ.

This is a very common pattern:

• 別の人 = another person
• 別の日 = another day
• 別の場所 = a different place

So 別のフォルダ does not mean a special kind of folder; it simply means a folder different from the current or usual one.

は marks the topic of the sentence.

So 大事なファイルは means something like:

• As for important files...
• When it comes to important files...

The sentence is not just identifying files; it is giving advice about them.

A useful way to understand the structure is:

• 大事なファイルは = as for important files
• 別のフォルダに保存しておくほうがいい = it is better to save them in a separate folder

Japanese often sets up the topic first and then comments on it.

Because に marks the place where something ends up or is put.

With 保存する, に is very natural for the destination/location of storage:

• フォルダに保存する = save it in a folder
• デスクトップに保存する = save it on the desktop

Here, the file ends up in that folder, so に fits well.

Using で would not sound natural here, because で usually marks the place where an action happens, not the destination/resulting location in this kind of expression.

〜ておく is a very common grammar pattern.

It often means:

• do something in advance
• do something and leave it that way
• do something for future convenience

So:

• 保存する = to save
• 保存しておく = to save in advance / to save and keep it that way

In this sentence, the nuance is:

• important files should be saved in a separate folder before it becomes a problem
• and kept there for safety or convenience

So おく adds more meaning than just 保存する.

Because here おく is acting as a helper verb, not the literal verb 置く meaning to put.

When 〜ておく is used as grammar, it is very often written in hiragana:

• 準備しておく
• 買っておく
• 保存しておく

This helps show that it means something like do in advance or do and leave as is, rather than the physical action put.

If it were the literal verb, kanji 置く would be more likely:

• 机の上に本を置く = put a book on the desk

Both are possible, but the nuance is different.

• 保存したほうがいい = You should save it. / It would be better to save it.
• 保存しておくほうがいい = It would be better to save it in advance / save it and keep it that way.

So the version with ておく adds the idea of preparation or future safety.

That makes sense here, because the sentence is giving practical advice about important files. It is not only saying save them; it is saying save them beforehand in a separate folder so they are safely kept there.

ほうがいい literally means the side/option is good, but in natural English it usually means:

• it is better to...
• you should...

So:

• 保存しておくほうがいい = it is better to save them in advance

Grammatically, 保存しておく is being treated like an option or course of action, and ほうがいい says that this option is preferable.

This pattern is very common for advice:

• 早く寝たほうがいい = You should go to bed early.
• メモしておいたほうがいい = You should write it down in advance.

A natural breakdown is:

• 大事なファイルは = as for important files
• 別のフォルダに = in a separate folder
• 保存しておく = save them in advance / save and keep them there
• ほうがいい = is better

So the whole sentence is built like this:

• As for important files, saving them in a separate folder is better.

A key point is that Japanese often puts the main verb near the end, and modifiers come before what they modify:

• 大事な modifies ファイル
• 別の modifies フォルダ

Also, Japanese often leaves out words that English would repeat. The sentence does not repeat them, but it is understood that the files are what should be saved.