Налогоплательщик проверяет налоговую декларацию.

Because it is the direct object of the verb проверяет (checks / is checking), so it goes into the accusative case.

The dictionary form is:

• налоговая декларация = tax declaration / tax return

In the accusative singular, a feminine adjective and noun change like this:

• налоговая → налоговую
• декларация → декларацию

So:

• Налогоплательщик проверяет налоговую декларацию.

means the taxpayer is checking the tax declaration.

Because it is the subject of the sentence, so it is in the nominative case.

Налогоплательщик is a masculine singular noun, and in the nominative singular many masculine nouns simply end in a consonant with no extra ending added.

So here:

• налогоплательщик = the taxpayer
• it is the person doing the action

Проверяет is the 3rd person singular present tense form of проверять.

So it means:

• he checks
• she checks
• it checks
• or, depending on context, is checking

In this sentence, the subject is налогоплательщик (taxpayer), so:

• налогоплательщик проверяет = the taxpayer checks / is checking

It can mean either one.

Russian does not usually make a strict grammatical distinction between:

• checks
• is checking

in the present tense the way English does.

So проверяет can mean:

• checks in a general or habitual sense
• is checking in a right-now sense

You decide from context.

The verb проверять is imperfective, which is the normal choice when you are talking about:

• a process
• a repeated action
• a general fact
• an action without focusing on completion

So проверяет presents the action as ongoing or general: the taxpayer is engaged in checking the declaration.

The perfective partner is usually проверить. Its future form is:

• проверит = will check / will have checked

Russian perfective verbs do not have a true present tense. So проверяет must be imperfective.

Because Russian has no articles.

So Russian often leaves it to context to tell you whether something is:

• a taxpayer or the taxpayer
• a tax declaration or the tax declaration

That means:

• Налогоплательщик проверяет налоговую декларацию.

could be interpreted in English in different ways depending on context, even though Russian uses no article words at all.

No. Russian word order is fairly flexible because the case endings show who is doing what.

The most neutral order here is:

• Налогоплательщик проверяет налоговую декларацию.
• Subject – Verb – Object

But other orders are possible for emphasis or focus, for example:

• Налоговую декларацию проверяет налогоплательщик.

That can sound like:

• It is the tax declaration that the taxpayer is checking, or
• The tax declaration is being checked by the taxpayer

So the original sentence is the most neutral, textbook-style version.

Russian often uses an adjective + noun structure where English might use a noun modifier.

So:

• налоговая декларация literally means something like tax-related declaration
• in natural English, that becomes tax declaration or tax return

Using налоговая here is the normal Russian way to describe the type of declaration.

Декларация налога would not be the normal standard phrase for this meaning.

It is a compound word:

• налог = tax
• connecting vowel -о-
• плательщик = payer

So налогоплательщик literally means:

• tax-payer
• that is, taxpayer

This is useful because Russian often builds formal vocabulary this way, especially in legal, financial, and administrative language.

Not necessarily.

In tax contexts, налоговая декларация is often translated more naturally as:

• tax return

even though the Russian word is literally declaration.

So depending on context, English may use:

• tax declaration
• tax return

Both can match the Russian phrase, but tax return is often the more natural everyday English equivalent.