Налоговую базу считают исходя из налогооблагаемого дохода.

Because it is the direct object of считают.

The verb считать can mean to count, to calculate, or to consider, depending on context. Here it means to calculate, so the thing being calculated is the direct object, and in Russian that normally goes into the accusative case.

• nominative: налоговая база
• accusative: налоговую базу

So:

• Налоговую базу считают... = They calculate the tax base... / The tax base is calculated...

Both words change because налоговая база is a feminine adjective + noun phrase, and the adjective must agree with the noun.

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

Russian often uses 3rd person plural with no stated subject when English would use:

• they
• people
• one
• or a passive construction

So считают literally looks like they calculate, but in context it often means:

• they calculate
• people calculate
• it is calculated

Here the most natural English translation is usually passive:

• The tax base is calculated based on taxable income.

This construction is especially common in formal, bureaucratic, legal, and instructional language.

Исходя is a verbal adverb (also called an adverbial participle or gerund in some Russian-learning materials).

It comes from the verb исходить and means something like:

• proceeding from
• based on
• starting from

In this sentence, исходя из functions almost like a fixed expression meaning based on.

So:

• исходя из налогооблагаемого дохода = based on taxable income

For many learners, it is easiest to memorize исходя из + genitive as a set phrase.

Because из requires the genitive case.

The phrase here is исходя из + genitive, and the key preposition is из, which normally takes the genitive.

So:

• nominative: налогооблагаемый доход
• genitive: налогооблагаемого дохода

That is why both words change:

• налогооблагаемый → налогооблагаемого
• доход → дохода

It is an adjective meaning taxable.

It agrees with дохода in:

• gender: masculine
• number: singular
• case: genitive

So the full phrase is:

• налогооблагаемого дохода = of taxable income

Even if you analyze its deeper history as connected to a verbal idea like subject to taxation, in this sentence it behaves just like a regular adjective modifying доход.

Yes, in normal usage, исходя из is followed by the genitive case.

Examples:

• исходя из ситуации = based on the situation
• исходя из опыта = based on experience
• исходя из закона = based on the law

So in your sentence:

• исходя из налогооблагаемого дохода

is exactly what you would expect.

Yes. A passive-style version would be very natural too, especially in formal language.

For example:

• Налоговая база рассчитывается исходя из налогооблагаемого дохода.

This means essentially the same thing:

• The tax base is calculated based on taxable income.

The difference is mostly stylistic:

• считают = indefinite-personal, literally they calculate
• рассчитывается = passive/reflexive style, is calculated

Russian official language often allows both kinds of wording.

Yes, the word order can be changed, because Russian word order is relatively flexible. But the given order is natural and neutral for formal written Russian.

Current order:

• Налоговую базу считают исходя из налогооблагаемого дохода.

This presents:

1. the object being discussed
2. the action
3. the basis for that action

A different order is possible, for example:

• Исходя из налогооблагаемого дохода считают налоговую базу.

This is still grammatical, but it shifts emphasis. It sounds more like Based on taxable income, they calculate the tax base.

In legal or explanatory prose, the original order is very natural.

Because this sentence describes a general rule or procedure, not a single completed act.

The imperfective verb считать is used for:

• habitual actions
• general processes
• instructions
• definitions
• repeated or standard procedures

That fits this sentence well: it explains how the tax base is calculated in general.

A perfective form would suggest a completed one-time result, which is not the main point here.

You know from the context.

The verb считать has several common meanings:

• to count
• to calculate
• to consider / to regard

In a financial or tax context, with налоговую базу as the object, the meaning is clearly to calculate.

So:

• считать налоговую базу = to calculate the tax base

If it meant consider, the object would usually be a person, idea, or proposition, for example:

• Я считаю его умным. = I consider him smart.
• Я считаю, что это правильно. = I think that this is correct.

Yes, very much so.

Words like:

• налоговая база
• налогооблагаемый доход
• исходя из

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

A native speaker would immediately recognize this as the style of:

• tax law
• accounting guidance
• official instructions
• government documents

So this is not everyday conversational Russian, but it is very normal in professional and official contexts.