Daňové přiznání bylo podáno v souladu se zákonem.

Because přiznání is a neuter singular noun. In Czech, adjectives must agree with the noun in gender, number, and case.

• daňový = masculine singular nominative
• daňová = feminine singular nominative
• daňové = neuter singular nominative/accusative, or plural in some contexts

So with přiznání (neuter), the correct form is daňové přiznání.

Přiznání comes from the verb přiznat / přiznat se, which often means to admit or to confess. So by itself, přiznání can mean admission or confession.

But in administrative and financial language, daňové přiznání is the standard term for a tax return or tax declaration.

So yes, the noun has a broader meaning in Czech, but in this phrase it is simply the normal official term.

This is a passive construction.

• bylo = past tense of být (to be), neuter singular
• podáno = passive participle of podat (to submit/file), also neuter singular

Together, bylo podáno means was submitted/filed.

It matches daňové přiznání, which is neuter singular, so both parts appear in neuter singular form.

The ending -o shows neuter singular in the passive participle.

Compare:

• dopis byl podán = the letter was submitted/filed (masculine)
• žádost byla podána = the application was submitted (feminine)
• přiznání bylo podáno = the return was submitted (neuter)

So podáno agrees with přiznání.

Yes, but the meaning and focus would change.

• Daňové přiznání bylo podáno... = The tax return was filed...
  This is passive and focuses on the action/result, not on who did it.

• Daňové přiznání podali... = Someone filed the tax return...
  This is active and would normally need a subject, either stated or understood.

In official style, Czech often prefers the passive when the person is unimportant or unspecified.

V souladu is a fixed expression meaning in accordance, in compliance, or in harmony.

Literally:

• soulad = harmony, agreement, conformity
• v souladu = in conformity / in accordance

So:

• v souladu se zákonem = in accordance with the law

This is just the normal Czech expression. You should learn v souladu s + instrumental as a set phrase.

The basic preposition is s (with), but Czech often uses se instead for easier pronunciation.

Before words beginning with certain consonant clusters or sounds, se sounds more natural:

• s bratrem
• se sestrou
• se zákonem

So se zákonem is simply the euphonic variant of s zákonem.

In this sentence, the full pattern is:

• v souladu s/se + instrumental

Because the preposition s/se requires the instrumental case.

The noun is:

• zákon = law

In the instrumental singular, it becomes:

• zákonem

So:

• se zákonem = with the law
• in this idiom, v souladu se zákonem = in accordance with the law

Yes, definitely. It is very common in formal, legal, and administrative Czech.

Useful related patterns:

• v souladu se zákonem = in accordance with the law
• v souladu s pravidly = in accordance with the rules
• v souladu s předpisy = in compliance with regulations
• v souladu s požadavky = in accordance with the requirements

A good pattern to memorize is:

• v souladu s/se + instrumental

Because Czech has no articles like English a/an/the.

So Daňové přiznání bylo podáno... can mean:

• The tax return was filed...
• A tax return was filed...

Which one is meant depends on context. In an official sentence like this, English usually translates it as the tax return.

Yes. Czech word order is more flexible than English because grammatical roles are shown by endings, not only by position.

This sentence could appear in different orders, for example:

• Daňové přiznání bylo podáno v souladu se zákonem.
• V souladu se zákonem bylo daňové přiznání podáno.

Both are grammatical, but they differ slightly in focus or emphasis.

The original order is neutral and natural in formal writing.

Yes. Podat is very common in official language for submit, file, or lodge a document.

Examples:

• podat žádost = submit an application
• podat odvolání = file an appeal
• podat daňové přiznání = file a tax return

In everyday English, file may sound specific to bureaucracy, and Czech podat works very similarly in these formal contexts.

It is in the nominative singular.

Even though English would think of it as the object of submit, in a Czech passive sentence the thing being acted on becomes the grammatical subject:

• Active: Někdo podal daňové přiznání.
• Passive: Daňové přiznání bylo podáno.

So in the passive version, daňové přiznání is nominative, and that is why the verb phrase agrees with it.