Запрещённое вложение нужно изъять.

Because it agrees with вложение, which is a neuter singular noun.

• вложение = neuter
• nominative singular neuter adjectives usually end in -ое or -ее
• so: запрещённое вложение

In other words, запрещённое means forbidden / prohibited, and its ending changes to match the noun it describes.

Вложение literally means something like an insertion, an enclosed item, or an attachment.

Depending on context, it can mean:

• an email attachment
• an enclosure in a letter or parcel
• something inserted inside something else

In modern everyday usage, many learners first meet it as attachment in email or file-related contexts.

In Russian, adjectives can sometimes function like nouns when the noun is obvious or when the adjective-noun phrase is treated as a unit.

Here, though, it is still simply an adjective describing вложение:

• запрещённое = prohibited
• вложение = attachment

So the full phrase is just prohibited attachment.

If the noun were omitted, Russian could sometimes still use the adjective substantively, but in this sentence the noun is present.

Нужно means it is necessary, one must, or needs to.

So:

• нужно изъять = it is necessary to remove/confiscate
• more naturally in English: must be removed / needs to be removed

This is a very common Russian impersonal construction:

• нужно + infinitive
• надо + infinitive

Examples:

• Нужно подождать. = It’s necessary to wait.
• Нужно проверить документ. = The document needs to be checked.

Because Russian often uses impersonal sentences where English would use something like it or a passive construction.

Запрещённое вложение нужно изъять literally feels like:

• Prohibited attachment — necessary to remove

Natural English:

• The prohibited attachment needs to be removed.
• The prohibited attachment must be confiscated/removed.

Russian does not need to say who must do it unless that information matters.

Because нужно is commonly followed by an infinitive.

Structure:

• нужно + infinitive

So:

• нужно изъять = it is necessary to remove/confiscate

Other examples:

• нужно сделать = it’s necessary to do
• нужно проверить = it’s necessary to check
• нужно отправить = it’s necessary to send

Изъять is more formal and specific than a simple everyday remove.

It often means:

• seize
• confiscate
• withdraw from use
• remove officially
• take out

So in different contexts it may sound like:

• confiscate the prohibited attachment
• remove the prohibited attachment
• withdraw the prohibited item

This verb is common in legal, official, technical, customs, and security language.

Because the choice of verb changes the tone and meaning.

• удалить = delete/remove, often used for files, text, or something you erase
• изъять = remove by taking out, seize, confiscate, withdraw officially

So if this is an official warning, customs message, moderation notice, or security-related statement, изъять sounds much more appropriate.

For example:

• Удалить файл = delete the file
• Изъять запрещённое вложение = remove/confiscate the prohibited attachment/item

It is in the nominative case.

Why? Because in this kind of impersonal sentence, the thing that needs to be removed is often left in the nominative, especially when Russian expresses something like:

• X нужно сделать
• X надо проверить
• X следует убрать

So the sentence is structured more around what requires action than around a normal active subject-object pattern.

A rough grammatical interpretation is:

• As for the prohibited attachment, it is necessary to remove it.

That is why you do not see accusative marking here.

Yes. Russian word order is flexible.

For example:

• Нужно изъять запрещённое вложение.
• Запрещённое вложение нужно изъять.

Both are possible.

The version with Запрещённое вложение first puts focus on the item itself:

• As for the prohibited attachment, it needs to be removed.

The version with Нужно изъять first sounds a little more action-focused:

• It is necessary to remove the prohibited attachment.

So the word order affects emphasis more than basic meaning.

Изъять is a perfective verb.

That matters because perfective verbs focus on a completed action or a single result.

Here, the idea is:

• the attachment must be taken out/removed completely

If Russian used an imperfective verb instead, it would sound more like an ongoing process or a general activity.

In instructions and official statements, perfective infinitives after нужно are very common when the goal is a completed result.

It is pronounced roughly as:

• за-пре-щё-нно-е

The ё is important because it shows both:

• the pronunciation yo
• the stress

So:

• запрещённое = with stress on -щё-

In printed Russian, ё is often written as е, so you may also see:

• запрещенное

But the pronunciation is still запрещённое, not запрещенное with a plain e sound.

It sounds fairly formal, official, or bureaucratic.

The reasons are:

• изъять is a formal verb
• the whole sentence is impersonal and concise
• it sounds like a notice, system message, legal statement, or administrative instruction

A more everyday version might use a different verb depending on context, for example:

• Нужно удалить вложение. = The attachment needs to be deleted/removed.
• Нужно убрать это вложение. = This attachment needs to be removed.

So the original sentence has a noticeably official tone.