taisetuna syorui ha kurippu de matomete oita hou ga ii.

Because 大切 is a な-adjective.

• Before a noun, a な-adjective takes な
• So 大切な書類 = important documents

Compare:

• 大切な書類 = important documents
• この書類は大切だ = this document is important

So the な is there because 大切 is directly modifying 書類.

は marks the topic of the sentence.

So 大切な書類は means something like:

• As for the important documents, ...

Even though the documents are still the thing being acted on, Japanese often uses は when the speaker wants to make that noun the topic or add a slight contrast.

This can imply something like:

• as for the important documents, you should clip them together
• maybe other papers are a different matter

If you used を, the sentence would sound more like a neutral statement about the direct object.

Here で marks the means or tool used to do something.

So:

• クリップで = with a clip / using a clip

That is different from を, which would mark a direct object.

In this sentence, the clip is not the thing being acted on. It is the tool used to hold the documents together.

まとめる has several related meanings, including:

• to gather together
• to put in order
• to bring together into one
• to summarize

In this sentence, because we are talking about documents and a clip, it means:

• to put together
• to gather into one bundle
• to clip together

So this is not the summarize meaning here. It is the physical grouping together meaning.

Because it is connecting to おく.

The pattern is:

• Vておく

So:

• まとめる → まとめて → まとめておく

The て-form is what lets one verb connect to another verb or helper expression like おく.

〜ておく often means:

• to do something in advance
• to do something and leave it that way

Here the main nuance is doing it beforehand as preparation.

So まとめておく suggests:

• clip the documents together ahead of time
• make sure they are already grouped together for later

That is an important nuance. The sentence is not just saying to clip them together somehow; it is saying it is better to do that in advance.

Because the common advice pattern is Vたほうがいい.

Here the relevant verb phrase is:

• まとめておく = to clip together in advance

Its plain past form is:

• まとめておいた

So the sentence uses:

• まとめておいたほうがいい

This does not mean the whole sentence is talking about the past. It is just the normal grammar pattern for giving advice.

For learners, it is best to remember:

• Vたほうがいい = should do V / had better do V

Formally, おいた is the past form of おく, but here it is part of the fixed advice pattern 〜たほうがいい.

So the sentence is not saying that someone already clipped the documents together in the past.

Instead, it means something like:

• it would be better to clip them together beforehand
• you should clip them together in advance

The sense of beforehand comes from 〜ておく, not from English-style past time.

〜たほうがいい is a very common way to give advice or make a recommendation.

It can mean:

• should
• had better
• it would be better to

So:

• まとめておいたほうがいい = it would be better to clip them together in advance

The tone is usually advisory, not a direct command, though it can still sound fairly strong depending on context and intonation.

Because ほう is treated like a noun meaning side, way, or option.

Very roughly, Vたほうがいい comes from the idea:

• the option of doing V is good

So が marks ほう as the thing that is いい.

You do not need to translate this literally every time, but understanding the structure can help:

• したほうがいい = the doing-it option is better

In practice, it is best learned as a set pattern.

The subject is omitted because Japanese often leaves out things that are clear from context.

Depending on the situation, this sentence could mean:

• you should clip the important documents together
• we should clip the important documents together
• the person handling them should do it

In many everyday contexts, listeners will naturally understand the omitted subject from the situation. That is very normal in Japanese.