Для заявки нужна электронная подпись, иначе документ не примут.

Because для requires the genitive case.

• заявка is the dictionary form (nominative)
• для заявки = for the application

So:

• заявка → nominative
• заявки → genitive singular

This is very common in Russian:

• для документа = for the document
• для подписи = for the signature
• для работы = for work

Because нужна agrees with подпись, which is a feminine singular noun.

Forms of нужен change like an adjective in the short form:

• нужен = masculine
• нужна = feminine
• нужно = neuter
• нужны = plural

Here, the thing that is needed is электронная подпись, and подпись is feminine, so Russian uses нужна.

Compare:

• Нужен документ. = A document is needed.
• Нужна подпись. = A signature is needed.
• Нужно письмо. = A letter is needed.
• Нужны документы. = Documents are needed.

The grammatical subject is электронная подпись.

Even though the sentence starts with для заявки, that part just means for the application and is not the subject.

A more literal structure is:

• For the application, an electronic signature is needed.

So:

• для заявки = prepositional phrase giving context
• электронная подпись = subject
• нужна = predicate agreeing with подпись

Yes, Электронная подпись нужна для заявки is also grammatical.

Russian word order is more flexible than English, and changing the order changes the focus slightly.

• Для заявки нужна электронная подпись puts the emphasis first on the application
• Электронная подпись нужна для заявки puts the emphasis first on the electronic signature

In this sentence, starting with для заявки sounds natural because it sets up the situation: as for the application / for the application, ...

It is nominative.

That is because it is the grammatical subject of the clause:

• электронная подпись нужна = an electronic signature is needed

This can be confusing for English speakers because English often uses need with an object, as in You need a signature. But Russian here uses a structure closer to:

• A signature is needed

So подпись stays in the nominative.

Иначе means otherwise or or else.

It introduces the consequence if the first condition is not met:

• Для заявки нужна электронная подпись, иначе документ не примут.
• An electronic signature is needed for the application, otherwise the document will not be accepted.

It is a very common connector in warnings, instructions, and official language.

Examples:

• Поторопись, иначе опоздаешь. = Hurry up, otherwise you’ll be late.
• Сохраните файл, иначе потеряете изменения. = Save the file, otherwise you’ll lose the changes.

This is a very common Russian pattern called an indefinite-personal construction.

The verb is in 3rd person plural with no stated subject:

• не примут = they won’t accept
• but it does not mean specific people you know
• it means people in charge / the relevant authorities / whoever handles it

In English, this is often translated more naturally as a passive:

• the document will not be accepted

Russian often prefers this active-looking plural form where English prefers the passive.

Similar examples:

• Вас ждут. = They are waiting for you. / You are expected.
• Здесь не курят. = People don’t smoke here. / No smoking here.
• Документы проверят завтра. = They will check the documents tomorrow. / The documents will be checked tomorrow.

Примут is the 3rd person plural future form of принять.

• infinitive: принять = to accept, to receive, to take
• future: приму, примешь, примет, примем, примете, примут

This is a perfective verb, so its future form is a simple future:

• примут = they will accept

The imperfective partner is принимать:

• принимают = they accept / are accepting
• будут принимать = they will be accepting

In this sentence, perfective примут is used because it refers to the result: whether the document gets accepted or not.

Not necessarily. Singular nouns in Russian are often used in a generic sense.

So документ не примут can mean:

• the document won’t be accepted
• a document like this won’t be accepted
• your submission won’t be accepted

Russian often uses the singular where English might also use the singular generically.

Depending on context, it could refer to:

• one specific document
• the application document as a whole
• the submitted paperwork in a general sense

Yes, that would be grammatical:

• Для заявки нужна электронная подпись, иначе документ не будет принят.

This is more explicitly passive: the document will not be accepted.

However, документ не примут is often more natural in everyday Russian, even in semi-official contexts. Russian frequently prefers the indefinite-personal construction over a passive form.

A rough comparison:

• документ не примут = more natural, common
• документ не будет принят = more formal, bureaucratic, written

Yes, the comma is correct and necessary here.

The sentence has two clauses:

• Для заявки нужна электронная подпись
• иначе документ не примут

Иначе connects them with the meaning otherwise, so Russian punctuation normally separates the clauses with a comma.

They can both sometimes be translated as application, but they are not always interchangeable.

• заявка often means a request, order, submission, booking, or application in an administrative/process sense
• заявление often means a formal written statement, declaration, or application document

In many real-life contexts, the exact choice depends on the institution and the type of form.

So для заявки suggests something like:

• for the application
• for the submission
• for the request

while для заявления would sound more like:

• for the formal application/document/statement

If this sentence comes from an official service or website, the noun used is probably the specific term that service prefers.

Yes. In official or administrative contexts, нужен / нужна / нужно / нужны often has a meaning close to:

• is required
• is necessary
• must be provided

So Для заявки нужна электронная подпись can feel stronger than just casual needed. In context, it means that an electronic signature is a necessary requirement for submitting or processing the application.