Исходя из срока подачи, налогоплательщик подаст налоговую декларацию завтра.

Here исходя из means something like based on, proceeding from, or taking into account.

So Исходя из срока подачи means based on the filing deadline / given the submission deadline.

A useful thing to know is that исходя из originally comes from the verb исходить and historically is a gerund-like form, but in modern Russian it is very often used almost like a fixed prepositional expression.

Because из normally requires the genitive case.

So:

• срок → nominative
• срока → genitive

That is why you get из срока.

Then подачи is also in the genitive because it depends on срок:

• срок подачи = the deadline/time limit for submission

So the whole phrase is:

• исходя из срока подачи
• literally: proceeding from the deadline of submission

Срок means term, deadline, time limit, or due date, depending on context.

Подача is a noun meaning submission or filing, from the verb подать (to submit, to file).

So:

• срок подачи = submission deadline
• in a tax context, very naturally filing deadline

This is a common Russian pattern:

• срок оплаты = payment deadline
• срок действия = period of validity / expiration term
• срок доставки = delivery time

Подаст is the 3rd person singular future of the perfective verb подать.

• подать = perfective, to submit/file as a completed act
• подаст = will submit

Russian uses the future of a perfective verb for a single completed future action.

So:

• налогоплательщик подаст декларацию завтра = the taxpayer will submit the tax return tomorrow

If you used the imperfective подавать, the meaning would be different:

• будет подавать could mean will be submitting, will submit repeatedly, or focus more on process rather than the completed act.

Since filing a declaration is usually viewed as one completed action, подаст is the natural choice.

Because налоговую декларацию is the direct object of подаст, so it is in the accusative case.

Base form:

• налоговая декларация = nominative singular

Accusative singular:

• налоговую декларацию

Both the adjective and noun change:

• налоговая → налоговую
• декларация → декларацию

This is a standard feminine singular accusative pattern for inanimate nouns.

Налогоплательщик means taxpayer.

It is in the nominative singular here because it is the subject of the sentence:

• налогоплательщик подаст = the taxpayer will submit

It is a masculine noun in form. In real usage, it can refer to a male taxpayer specifically, or more generally to a taxpayer as a legal/administrative category, depending on context.

Because Russian has no articles.

Russian does not have separate words for a/an and the. Whether something is definite or indefinite is understood from context.

So:

• налогоплательщик can mean a taxpayer or the taxpayer
• налоговая декларация can mean a tax declaration or the tax declaration / tax return

In your sentence, English might naturally translate it as the taxpayer and the tax return, but Russian itself does not mark that with an article.

Yes. Russian word order is much more flexible than English word order because case endings show grammatical relationships.

Your sentence is:

• Исходя из срока подачи, налогоплательщик подаст налоговую декларацию завтра.

This order is fine, and it gives a neutral-to-formal feel.

You could also see variants like:

• Налогоплательщик подаст налоговую декларацию завтра, исходя из срока подачи.
• Завтра налогоплательщик подаст налоговую декларацию...

But changing the order changes the emphasis a little:

• sentence-initial завтра emphasizes tomorrow
• sentence-initial исходя из срока подачи foregrounds the reasoning/background

So the word order is not random; it helps manage focus and style.

Because this opening phrase is treated as a detached introductory/adverbial expression.

Historically, исходя из is connected with a gerund-like form, and gerundial phrases are normally set off by commas in Russian.

That said, исходя из is also often treated in modern Russian almost like a fixed preposition, and punctuation with such expressions can sometimes be less straightforward in real usage. But in a sentence like this, the comma is very understandable and expected by many writers.

So for a learner, the practical answer is:

• the opening исходя из... phrase is separated by a comma as an adverbial modifier.

It is understandable, but many native speakers might find it a bit formal or slightly awkward depending on the exact intended meaning.

The main issue is исходя из срока подачи. It sounds bureaucratic and can feel a little unnatural if you simply mean given the deadline or judging by the deadline.

Depending on the intended meaning, more idiomatic alternatives might be:

• Судя по сроку подачи, налогоплательщик подаст налоговую декларацию завтра.
  = Judging by the filing deadline...

• Учитывая срок подачи, налогоплательщик подаст налоговую декларацию завтра.
  = Considering the filing deadline...

• Из-за срока подачи налогоплательщик подаст налоговую декларацию завтра.
  = Because of the deadline, the taxpayer will submit... tomorrow
  (this changes the nuance to cause/pressure)

Also, if the idea is simply that the deadline is tomorrow and therefore the taxpayer will file tomorrow, Russian might express that even more directly.

So yes, the sentence is grammatical enough to study, but it is worth noticing that исходя из often sounds quite official.

This is a classic Russian aspect question.

• подать = perfective
  Focus on the action as a whole, as a completed event

• подавать = imperfective
  Focus on process, repetition, or general activity

In this sentence, the taxpayer is expected to submit the declaration one time, as a completed action, so подать is the right choice:

• подаст = will submit

Compare:

• Он подаст декларацию завтра.
  = He will submit the declaration tomorrow.

• Он будет подавать декларацию завтра.
  = He will be in the process of submitting it tomorrow / he will be submitting it tomorrow.
  This is less natural unless you specifically want to emphasize process.

• Он подаёт декларацию каждый год.
  = He files a declaration every year.
  Here the imperfective is natural because it describes repeated action.

No. Завтра is flexible in position.

You can say:

• ...подаст налоговую декларацию завтра
• Завтра налогоплательщик подаст налоговую декларацию
• Налогоплательщик завтра подаст налоговую декларацию

All of these are possible.

The differences are mostly about emphasis:

• end position can sound neutral
• sentence-initial завтра highlights the time
• placing завтра near the verb can make the timing feel especially relevant to the action

So the sentence-final завтра is fine, but not the only option.