Мне нужно отнести заявление в отдел кадров до обеда.

Because нужно is used in an impersonal construction.

Russian does not say this the same way English says I need to.... Instead, it is closer to:

To me, it is necessary to...

So:

• мне = to me in the dative case
• нужно = necessary / needed
• infinitive = the action that needs to be done

That is why мне is correct here, not я.

A similar pattern appears in:

• Мне надо идти. = I need to go.
• Ему нужно позвонить. = He needs to call.

Нужно means something like it is necessary, it is needed, or one needs to.

In this sentence, it combines with the infinitive отнести to express obligation or necessity:

• мне нужно отнести... = I need to take/deliver...

It is a very common everyday way to say that something has to be done.

A few related words:

• нужно = necessary, needed
• надо = also means need to / have to, often very similar in everyday speech
• должен / должна = must, am supposed to, more personal and direct

So мне нужно отнести заявление sounds natural and neutral.

Because отнести is the better choice for a single completed action with a destination.

Compare:

• нести = to be carrying, to carry in one direction, often focusing on the process
• отнести = to take/carry something somewhere and deliver it there

In this sentence, the speaker is talking about a specific task that needs to be completed: taking the application to HR. So Russian uses the perfective verb отнести.

This is especially common after нужно / надо when talking about one concrete thing that must be done:

• Мне нужно написать письмо.
• Мне нужно купить хлеб.
• Мне нужно отнести заявление.

If you used нести, it would sound more like focusing on the ongoing action of carrying rather than the completed errand.

The prefix от- often gives the idea of taking something away or to a destination away from the starting point.

So:

• нести = carry
• отнести = carry/take away, take over, deliver

In practice, отнести заявление в отдел кадров means to take the document over there and leave/deliver it in the HR department.

In many contexts, English might translate отнести as:

• take
• bring
• deliver

The exact English choice depends on perspective, but in Russian the important idea is that the item is transported to its destination.

It is in the accusative, but for an inanimate neuter singular noun, the accusative looks exactly like the nominative.

So:

• nominative: заявление
• accusative: заявление

Since отнести takes a direct object, заявление is indeed the object of the verb, but its form does not visibly change.

This is very normal in Russian.

For comparison:

• я читаю письмо — письмо is accusative, but looks like nominative
• я вижу стол — стол is accusative, same as nominative
• but animate masculine nouns do change differently, for example:
  • я вижу брата

Here в means into / to a place, because the sentence describes movement toward a destination.

After в:

• accusative is used for motion into/to
• prepositional is used for location in

So:

• в отдел кадров = into/to the HR department → motion → accusative
• в отделе кадров = in the HR department → location → prepositional

That is why the sentence has отдел, not отделе.

Compare:

• Я иду в банк. = I am going to the bank.
• Я в банке. = I am in the bank.

Отдел кадров is a fixed expression meaning personnel department or HR department.

Literally, it is structured like:

• отдел = department
• кадров = of personnel/staff

So кадров is in the genitive plural.

This is a common Russian pattern:

• центр города = city center
• министр иностранных дел = minister of foreign affairs
• отдел кадров = department of personnel / HR department

You should learn отдел кадров as a set phrase. It is one of the standard ways to say HR in Russian.

The preposition до requires the genitive case.

So:

• обед = lunch
• до обеда = before lunch / until lunch

This is a very common pattern:

• до вечера = before evening
• до утра = before morning
• до понедельника = before Monday

So обеда is simply the genitive singular form of обед.

It can suggest either, depending on context, and that is normal.

In this sentence, до обеда most naturally means:

• before lunch
• by lunchtime
• before the lunch break

It gives a deadline.

Russian до often covers both the idea of up to and before. In practical translation here, before lunch is the most natural English choice.

Yes. Russian word order is more flexible than English word order.

The neutral order here is:

Мне нужно отнести заявление в отдел кадров до обеда.

But other orders are possible if you want to emphasize something:

• До обеда мне нужно отнести заявление в отдел кадров.
  Emphasizes the deadline.

• Заявление мне нужно отнести в отдел кадров до обеда.
  Emphasizes the application.

• В отдел кадров мне нужно отнести заявление до обеда.
  Emphasizes the destination.

The original version sounds natural and neutral, but Russian often moves elements around for focus or style.

Yes. Надо and нужно are often interchangeable in everyday speech.

So this would also sound natural:

Мне надо отнести заявление в отдел кадров до обеда.

Very roughly:

• нужно can sound a little more neutral or formal
• надо can sound a little more conversational

But in many real situations, the difference is small.

If you said:

• Я должен / Я должна отнести заявление...

that would sound more like:

• I must
• I am supposed to
• I am obliged to

That version is more personal and stronger than мне нужно.

It is really an impersonal sentence structure.

Russian often expresses necessity without a normal grammatical subject. Instead of saying I as the subject, it uses:

• a person in the dative: мне
• a word like нужно or надо
• an infinitive: отнести

So the structure is:

[to whom] + [necessary] + [what to do]

That is why this sentence feels subjectless grammatically, even though the meaning clearly refers to me. This kind of construction is extremely common in Russian.