Доначисление началось после камеральной проверки.

Because the subject is доначисление, and доначисление is a neuter singular noun.

In the Russian past tense, verbs agree with the subject in gender and number:

• masculine: начался
• feminine: началась
• neuter: началось
• plural: начались

So here:

• доначисление = neuter singular
• therefore началось = neuter singular past tense

The -сь is the reflexive marker, the short form of -ся.

Here the verb is начаться, not начать.

• начать = to begin/start something
• начаться = to begin/start (for something itself)

Compare:

• Компания начала проверку. = The company started the audit.
• Проверка началась. = The audit started / began.

So in your sentence, доначисление началось means the additional assessment began. The thing itself began, so Russian uses начаться.

It is a noun, specifically a deverbal noun: a noun made from a verb.

It comes from the verb доначислить / доначислять, which is used in formal financial or tax language for assessing additional amounts, charging extra, or making an additional assessment.

The ending -ение is very common in Russian for nouns formed from verbs, especially in formal style:

• начислить → начисление
• доначислить → доначисление

So grammatically, доначисление behaves like an ordinary neuter noun, even though semantically it refers to an action or result.

Because the preposition после requires the genitive.

So:

• после чего? = after what?
• после проверки = after the audit/inspection

In your sentence:

• после
  • камеральной проверки

Both words are in the genitive singular:

• камеральная → камеральной
• проверка → проверки

This is a very common pattern:

• после работы = after work
• после урока = after the lesson
• после проверки = after the inspection

Good question, because проверки by itself can be ambiguous:

• проверки can mean of the audit (genitive singular)
• or audits (nominative plural / accusative plural in some contexts)

But here the adjective камеральной makes it clear.

• камеральной is genitive singular feminine
• so проверки must also be genitive singular

So the phrase means after the desk audit / after the office-based audit, not after desk audits.

Камеральная проверка is a fixed official term, especially in tax and legal language.

It usually means a desk audit, office audit, or document-based audit carried out from submitted records, rather than by visiting the business in person.

So камеральный is not a common everyday adjective in ordinary conversation. It belongs mostly to bureaucratic, legal, accounting, and tax language.

A learner will often meet it in contrasts like:

• камеральная проверка = desk/office audit
• выездная проверка = on-site audit / field audit

Not really. It is grammatical and natural, but it sounds formal, bureaucratic, and specialized.

Words like:

• доначисление
• камеральная проверка

strongly suggest tax, accounting, or legal-administrative language.

A native speaker would expect this kind of sentence in:

• tax notices
• legal documents
• accounting reports
• official correspondence

So this is good Russian, but it is not casual conversational wording.

Yes. Russian word order is flexible, and changing it usually changes emphasis, not the core meaning.

Your sentence has a fairly neutral structure:

• Доначисление началось после камеральной проверки.

You could also say:

• После камеральной проверки началось доначисление.

That version puts more focus on the time circumstance: after the desk audit.

So both are possible, but the original sounds like a straightforward statement with доначисление as the topic.

Because после камеральной проверки is just a prepositional phrase, not a clause.

It literally means:

• after
  • the desk audit

Russian does not need a separate verb here. English also would not say after was the audit; it simply says after the audit.

So the full sentence structure is:

• Доначисление = subject
• началось = verb
• после камеральной проверки = time phrase

Yes. In most cases, both work.

• началось = began
• началось = started

In English, began can sound a bit more formal, and started can sound a bit more neutral or everyday, but both are normal translations.

That said, the Russian sentence itself is formal because of the vocabulary, so in context began may sometimes match the tone a little better.