Podle zákona podá poplatník daňové přiznání.

Because podle requires the genitive case.

The noun zákon is the dictionary form, which is nominative singular. After podle, it changes to genitive singular:

• zákon = law
• podle zákona = according to the law / under the law

This is something you simply have to learn with the preposition: podle + genitive.

It most naturally means according to the law or under the law.

In legal or official language, podle zákona often means as required or provided by law, not just in the opinion of the law. So in this kind of sentence, it can sound a bit more formal than everyday English according to the law.

This is a very common point of confusion.

Podá is the 3rd person singular form of the perfective verb podat. With perfective verbs in Czech, the so-called non-past form usually refers to the future, not the present.

So:

• podat → perfective → podá = will submit / will file
• podávat → imperfective → podává = submits / is submitting / submits regularly

So although podá does not look like an English future with will, it normally has future meaning.

They are an aspect pair:

• podat = perfective
  Focuses on a completed act: submitting something once, successfully, as a whole.
• podávat = imperfective
  Focuses on process, repetition, habit, or ongoing action.

In this sentence, podat is used because filing a tax return is seen as a single completed action.

So:

• poplatník podá daňové přiznání = the taxpayer will file the return
• poplatník podává daňové přiznání = the taxpayer is filing / files the return / files returns regularly, depending on context

Poplatník is in the nominative singular. It is the subject of the sentence.

It comes after the verb because Czech word order is much freer than English word order. Czech uses case endings more than position to show who is doing what.

So this is still clear:

• podá = will file
• poplatník = the taxpayer, nominative subject

Putting poplatník after the verb is perfectly normal. The sentence begins with Podle zákona to set the legal frame first, and then gives the action.

Here it is in the accusative singular, because it is the direct object of podá.

The taxpayer files what?
→ daňové přiznání

A useful thing to know is that for many neuter nouns, the nominative and accusative singular look the same. That is why přiznání does not visibly change here.

Because the adjective must agree with the noun.

Přiznání is a neuter singular noun, so the adjective must also be in the neuter singular form:

• daňové přiznání = tax return

Compare:

• daňový doklad = tax document
  because doklad is masculine
• daňová povinnost = tax obligation
  because povinnost is feminine
• daňové přiznání = tax return
  because přiznání is neuter

So daňové is there because of grammatical agreement.

Grammatically, podá is a future meaning: will file / will submit.

But in legal or official language, this kind of future can also function a bit like a rule or requirement. So depending on context, the sentence may feel close to:

• the taxpayer shall file a tax return
• the taxpayer must file a tax return

That is a nuance of formal/legal style. The exact English translation depends on the context the sentence comes from.

Because Czech has no articles.

English needs words like the and a/an, but Czech normally does not. Whether something is definite or indefinite is understood from context.

So:

• poplatník can mean the taxpayer or sometimes just a taxpayer, depending on context
• daňové přiznání can mean a tax return or the tax return, again depending on context

If Czech wants to be more specific, it can use other words, such as ten for that/the specific one, but there is no regular article system like English.

You can, but the emphasis changes.

The original sentence:

• Podle zákona podá poplatník daňové přiznání.

puts Podle zákona first, so the legal basis is the frame for the whole statement.

If you say:

• Poplatník podá daňové přiznání podle zákona.

it may sound more like the taxpayer will file the tax return according to the law, where podle zákona is attached more closely to the action itself.

So the changed version is not automatically wrong, but it does not highlight the same thing in the same way. In Czech, word order often affects focus and emphasis, not just grammar.