Заушник нужно проверить.

It is in the accusative case, because it is the direct object of проверить.

The reason it looks exactly like the dictionary form is that заушник is a masculine inanimate noun. For masculine inanimate nouns, the accusative singular is the same as the nominative singular.

So:

• nominative: заушник
• accusative: заушник

Even though the form does not change, the function in the sentence is still object, not subject.

Нужно means it is necessary, needful, or more naturally in English, needs to / must / should depending on context.

In this sentence, нужно + infinitive forms an impersonal construction of necessity:

• нужно проверить = it is necessary to check
• more natural English: needs to be checked

A key point: нужно does not change for person or number here. It is not like an ordinary verb such as проверяю / проверяет.

Russian often uses impersonal sentences where English uses a dummy subject like it, or a passive construction.

So Russian says:

• Заушник нужно проверить

while English often says:

• The earhook needs to be checked
• or It is necessary to check the earhook

Russian does not need a word corresponding to English it here. The sentence simply states that the action is necessary.

Проверить is the perfective infinitive. It presents the action as one complete check.

That fits very well in sentences about instructions, tasks, or single required actions:

• Заушник нужно проверить = the item needs one checking/completed check

If you said нужно проверять, that would usually suggest a more general, repeated, or habitual idea:

• Заушник нужно проверять регулярно = it needs to be checked regularly

So in your sentence, проверить is used because the speaker means a specific completed check.

Yes, Нужно проверить заушник is also correct.

Russian word order is more flexible than English word order, and changing the order often changes the focus or emphasis, not the basic meaning.

A rough comparison:

• Нужно проверить заушник
  more neutral: The earhook needs to be checked

• Заушник нужно проверить
  puts extra attention on заушник: As for the earhook, that needs to be checked

So fronting заушник can make it sound more contrastive or topic-like.

Not grammatically, no.

In English, the most natural translation is often passive:

• The earhook needs to be checked

But in Russian, the structure is not a passive verb form. It is an impersonal necessity construction:

• нужно + infinitive

So structurally, Russian is closer to:

• It is necessary to check the earhook

English just often prefers the passive wording in this kind of sentence.

Yes. Заушник надо проверить is perfectly natural.

In many everyday situations, надо and нужно are very close in meaning. Both can express necessity.

Very roughly:

• надо often sounds a bit more conversational
• нужно can sound a bit more neutral or slightly more formal

But in a sentence like this, the difference is usually small, and both are common.

Yes, depending on context.

Russian does not state who must perform the action. The sentence only says that the checking is necessary.

So in English, the best translation could vary:

• The earhook needs to be checked
• We need to check the earhook
• The earhook should be checked

The exact English version depends on what sounds most natural in the situation and whether you want to make the doer explicit.

Russian has no articles.

So заушник can mean:

• the earhook
• an earhook

Context tells you which one is intended.

In a real situation, this sentence often comes across as the earhook needs to be checked, because the speaker usually has a specific item in mind. But Russian itself does not mark that with an article.