Налоговый период указан в налоговой декларации.

Because период is the grammatical subject of the sentence.

In Налоговый период указан в налоговой декларации, the structure is basically:

• Налоговый период = the tax period
• указан = is indicated / is specified
• в налоговой декларации = in the tax declaration

So период is the thing being talked about, which is why it stays in the nominative.

Указан is the short-form passive participle of the verb указать, meaning to indicate, to specify, or to state.

So:

• указать = to indicate / specify
• указанный = indicated / specified, as a full adjective-like form
• указан = indicated / specified, in the short form used in predicate position

In this sentence, указан works like is specified or is indicated.

A very literal breakdown would be:

• Налоговый период указан = The tax period is indicated

This short form is very common in formal and written Russian.

Because Russian uses two different forms here:

• Full form: указанный
• Short form: указан

The full form usually behaves like an adjective and modifies a noun directly:

• указанный период = the specified period

The short form is commonly used as the main predicate of the sentence:

• Период указан = The period is specified

So in your sentence, указан is correct because it is part of the predicate, not directly describing the noun inside a noun phrase.

Because it agrees with период, which is masculine singular.

Short-form passive participles change for gender and number:

• masculine: указан
• feminine: указана
• neuter: указано
• plural: указаны

Since период is masculine singular, the form must be указан.

For example:

• Дата указана = The date is specified
• Место указано = The place is specified
• Данные указаны = The data are specified

Because after в when expressing location or where something is written/found, Russian usually uses the prepositional case.

Here the meaning is:

• in the tax declaration

So:

• декларация → в декларации
• налоговая декларация → в налоговой декларации

Both the adjective and noun change to the prepositional singular feminine form:

• налоговая → налоговой
• декларация → декларации

Because it is the same adjective in two different grammatical forms.

1. Налоговый период

   • период is masculine nominative singular
   • so the adjective is налоговый

2. в налоговой декларации

   • декларации is feminine prepositional singular
   • so the adjective becomes налоговой

Russian adjectives must agree with the nouns they describe in:

• gender
• number
• case

So the adjective changes form depending on the noun.

Often, yes, in many contexts налоговая декларация is translated as tax return or tax declaration.

A few notes:

• декларация is more literally declaration
• in tax contexts, English often prefers tax return
• Russian official language frequently uses налоговая декларация

So depending on context, this sentence could be translated as:

• The tax period is indicated in the tax declaration
• The tax period is specified in the tax return

Both can be natural.

Yes, but the meaning shifts slightly.

Compare:

• Налоговый период указан в налоговой декларации. = The tax period is specified in the tax declaration.

  • This sounds like a statement about the finished result or stated fact.

• Налоговый период указывается в налоговой декларации. = The tax period is specified/is to be specified in the tax declaration.

  • This sounds more general, procedural, or instructional.

So:

• указан = result/state
• указывается = general rule/process/habitual action

In formal documents, both are possible, but they are not exactly the same.

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

The neutral order here is:

• Налоговый период указан в налоговой декларации.

But you could also say:

• В налоговой декларации указан налоговый период.

This version puts more emphasis on where it is indicated.

Both are grammatical. The difference is mostly about emphasis and information structure, not basic meaning.

Not really. The sentence is focused on the fact that the tax period is specified, not on who specified it.

Russian often uses passive-like constructions like this when the agent is unimportant, unknown, or obvious from context.

So the sentence does not mean:

• someone specific is being highlighted as the writer

Instead, it simply states the information as it appears in the document.

Yes, very much so.

Words like:

• налоговый период
• налоговая декларация
• указан

belong to formal, administrative, legal, or bureaucratic Russian.

In everyday conversation, people might use shorter or less official phrasing depending on context, but in forms, instructions, and official documents, this sentence sounds completely normal.

The main word stress is:

• нало́говый
• пери́од
• указа́н
• в нало́говой
• деклара́ции

So roughly:

нало́говый пери́од указа́н в нало́говой деклара́ции

For an English speaker, the biggest thing is not to stress the wrong syllable in:

• нало́говый
• указа́н
• деклара́ции

Russian stress is unpredictable, so it is worth learning these individually.