husinna roguin ni tuite toiawasemadoguti ni renrakusuru.

Because 不審 is a na-adjective noun-like word, not an i-adjective.

• 不審 = suspicion / suspiciousness
• before a noun, it becomes 不審な
• so 不審なログイン = a suspicious login

This is the same pattern as:

• 静か → 静かな部屋
• 大切 → 大切な人

So here, な is needed because ログイン is the noun being modified.

It means something like a suspicious login or a login that seems abnormal or possibly unauthorized.

The nuance is not just strange in a casual sense. It suggests:

• something that raises concern
• something potentially unsafe
• something that may need investigation

In security or website/app settings, 不審なログイン is a very common phrase for suspicious account access.

について means about, regarding, or concerning.

So:

• 不審なログインについて = about the suspicious login

It attaches to a noun phrase and marks the topic of the action that follows.

Examples:

• 日本について勉強する = study about Japan
• この件について話す = talk about this matter

In your sentence, it tells us what the contact is about.

They are doing two different jobs.

1. について

   • this is a fixed grammar pattern meaning about/regarding
   • the に here is part of that expression

2. 問い合わせ窓口に

   • this に marks the target/recipient of 連絡する
   • in other words, contact the inquiry desk/contact point

So the sentence has:

• 不審なログインについて = regarding the suspicious login
• 問い合わせ窓口に連絡する = contact the support desk

Even though both contain に, they are not the same grammatical role.

問い合わせ窓口 is a compound noun meaning something like:

• inquiry desk
• contact point
• customer support contact
• help desk

Breaking it down:

• 問い合わせ = inquiry, question, contact for information
• 窓口 = window/counter/contact point

In modern business/website Japanese, 窓口 often means the department or channel you should contact, not just a physical counter.

So 問い合わせ窓口 is the place/person/system you contact when you need help.

Both can work in related situations, but they are not exactly the same.

• 連絡する = to contact
• 問い合わせる = to inquire / ask for information

Here, the sentence focuses on the action of getting in touch with the proper support channel:

• 問い合わせ窓口に連絡する = contact the inquiry desk

That sounds very natural in instructions.

If you used 問い合わせる, the focus would be more on asking about the issue itself. For example:

• 不審なログインについて問い合わせる = inquire about the suspicious login

That is also possible, but the original sentence emphasizes who to contact.

The main verb is 連絡する = to contact.

Everything before it gives extra information:

• 不審なログインについて = about the suspicious login
• 問い合わせ窓口に = to the inquiry desk/contact point

So the basic skeleton is:

• 問い合わせ窓口に連絡する
• Contact the support desk

And 不審なログインについて adds what the contact is concerning.

Yes. Japanese often omits the subject when it is understood from context.

This sentence does not explicitly say:

• who should contact them
• whose login it is

Depending on context, it could mean things like:

• Contact the support desk about a suspicious login.
• If there is a suspicious login, contact the help desk.
• You should contact the inquiry desk regarding suspicious login activity.

This kind of subject omission is very normal in Japanese, especially in instructions, menus, or security guidance.

The plain form is often used in:

• headings
• menu items
• instruction lists
• policy or manual style writing

So 連絡する here can sound like:

• a step in a procedure
• a label for an action
• a concise instruction

If you wanted a more direct polite instruction, you could say:

• 不審なログインについて問い合わせ窓口に連絡してください。

That would mean Please contact the inquiry desk about the suspicious login.

So the plain form is not rude by itself here; it is just more concise and manual-like.

No. Normal Japanese writing would usually not include spaces like this.

It would normally appear as:

不審なログインについて問い合わせ窓口に連絡する。

Spaces are often added only in teaching materials to make the structure easier for learners to see.

To some extent, yes. Japanese word order is more flexible than English, as long as the particles make the roles clear.

The most natural version here is:

不審なログインについて問い合わせ窓口に連絡する。

You could also move parts around for emphasis, for example:

• 問い合わせ窓口に、不審なログインについて連絡する。

This is still understandable, because:

• について marks the topic
• に marks the contact target

But the original order is smoother and more standard in a neutral written context.

As a standalone full sentence, 連絡する。 in plain dictionary form can sound a bit like:

• a note
• a heading
• an instruction step
• a policy statement

So it is natural in the right context, such as:

• app instructions
• website security guidance
• a procedure list

But in normal conversation, you would more often hear something like:

• 問い合わせ窓口に連絡してください。
• 問い合わせ窓口に連絡します。
• 問い合わせ窓口に連絡したほうがいいです。

So the original is natural, but it sounds like written instructional Japanese rather than everyday spoken conversation.