Коммерческую партию нужно декларировать.

Because they are in the accusative case.

• партия is a feminine noun.
• In this sentence, it is the direct object of декларировать (to declare), so it has to be in the accusative.
• The adjective коммерческая also changes to match the noun in gender, number, and case.

So:

• nominative: коммерческая партия
• accusative: коммерческую партию

That is why the sentence has Коммерческую партию нужно декларировать.

No. Here партия does not mean a political party.

In commercial, customs, and shipping contexts, партия often means:

• batch
• lot
• consignment
• shipment

So коммерческая партия means something like a commercial shipment or commercial consignment of goods.

Нужно is an impersonal predicate word meaning:

• it is necessary
• it is required
• must / need to

So нужно декларировать literally means:

• it is necessary to declare

This structure is very common in Russian:

• нужно делать = it is necessary to do
• нужно заполнять форму = you need to fill out the form
• нужно декларировать = must be declared / need to declare

Russian often uses this impersonal structure where English might use must, have to, or even a passive like must be declared.

That is normal in Russian.

The sentence is impersonal, so it focuses on the requirement, not on the person responsible. English does this too sometimes, especially in formal language: It must be declared.

Who exactly must do it depends on context:

• the importer
• the exporter
• the traveler
• the customs declarant
• the company handling the goods

So the Russian sentence leaves the subject unspecified on purpose.

Because after нужно, Russian often uses the imperfective infinitive when talking about a general rule, obligation, or procedure.

So нужно декларировать means something like:

• it is necessary to declare
• it has to be declared
• declaration is required

The imperfective here does not mean the action is repeated. It often just presents the action in a general way.

You could sometimes see a perfective verb in similar contexts, but it would usually put more emphasis on completing the action successfully. The imperfective is the more neutral and common choice for rules and requirements.

Russian word order is flexible, so putting Коммерческую партию first helps emphasize the object or make it the topic of the sentence.

This order is natural, especially in formal or informational contexts.

Compare:

• Коммерческую партию нужно декларировать.
• Нужно декларировать коммерческую партию.

Both are grammatical. The difference is mainly in focus and style, not basic meaning.

The version with the object first can sound a bit more like an official statement about that specific type of goods.

Yes. Russian has no articles like a and the, so context decides.

Коммерческую партию could mean:

• a commercial shipment
• the commercial shipment
• commercial goods / a commercial consignment

In translation, English has to choose an article, but Russian does not mark that directly.

Yes, it sounds fairly formal and official.

Words like:

• коммерческая партия
• декларировать

are typical of:

• customs language
• legal or administrative language
• shipping and trade contexts
• official instructions

In casual speech, people might use simpler wording depending on context, but this sentence sounds perfectly natural in official or bureaucratic Russian.

A helpful stress pattern is:

коммерчЕскую пАртию нУжно декларИровать

Approximate pronunciation:

kammir-CHE-sku-yu PAR-ti-yu NOOZH-na di-kla-REE-ra-vat'

A couple of useful notes:

• что/чк/чн type spellings can sometimes surprise learners, but here коммерческую is pronounced roughly as written.
• нужно is pronounced with stress on у: нУжно
• декларировать has stress on ри: декларИровать

Yes. That is a good natural translation.

Other good translations include:

• The commercial shipment must be declared.
• A commercial shipment has to be declared.
• Commercial goods must be declared.

Which one is best depends on context, but they all match the Russian structure well.