Držitel psa musí ohlásit identifikační údaje.

Because psa is in the genitive singular here.

In držitel psa, the noun držitel often takes the genitive, like holder of a dog or owner of a dog. So:

• pes = dog (basic dictionary form, nominative)
• psa = of a dog / dog’s, depending on context

So držitel psa literally works like holder of a dog.

Držitel means something like holder, possessor, or in legal/official language often registered holder.

It is similar to majitel (owner), but not always identical in nuance:

• majitel = owner in a general sense
• držitel = holder/possessor, often more formal or legal/administrative

In official texts, držitel psa may be used instead of majitel psa because it sounds more formal and legal.

Musí is the 3rd person singular present tense of muset (must / have to).

It is singular because the subject is držitel = the holder, which is singular.

So:

• já musím = I must
• ty musíš = you must
• on / ona / ono musí = he / she / it must
• držitel musí = the holder must

Ohlásit is the perfective infinitive, and that fits well after musí when talking about a required completed action.

Compare:

• ohlásit = to report, to declare, to notify once / as a completed act
• ohlašovat = to be reporting, to report repeatedly or as an ongoing process
• hlásit = to report, announce, signal; a more general verb

In an official rule or obligation, Czech often prefers the perfective form when the point is that the action must be carried out:

• musí ohlásit = must report / must notify

It is in the accusative plural, because it is the direct object of ohlásit.

The verb asks ohlásit co? = report what?

Answer: identifikační údaje

A useful detail:

• údaj is masculine inanimate
• for many masculine inanimate nouns, the nominative plural and accusative plural are the same

So údaje can be both nominative plural and accusative plural. Here it is accusative because it is the object of the verb.

In this context, plural is natural because identification details/data are usually understood as a set of pieces of information, not just one item.

So identifikační údaje means something like:

• identification details
• identifying information
• identification data

Singular identifikační údaj would mean one specific identifying piece of information, which is less likely in this kind of official sentence.

It usually refers to official identifying information, depending on context. For a dog, that could mean things like:

• chip number
• registration number
• owner/holder details
• breed, date of birth, or similar official data

In legal/administrative Czech, údaje often means details, data, or information rather than casual facts.

Czech does not have articles like English a and the.

So a phrase like držitel psa can mean:

• a dog holder
• the dog holder
• holder of a dog

The exact meaning depends on context.

That is completely normal in Czech. Learners often want to add an article mentally, but Czech simply does not use them.

Yes. Czech word order is more flexible than English because grammatical roles are shown largely by case endings.

The neutral order here is:

• Držitel psa musí ohlásit identifikační údaje.

But other orders are possible, for example for emphasis:

• Identifikační údaje musí držitel psa ohlásit.
• Držitel psa identifikační údaje musí ohlásit.

The original version sounds natural and neutral, especially in formal or official language.

Yes, in this sentence it is best understood as genitive singular.

It is true that for animate masculine nouns, genitive singular and accusative singular can look the same:

• pes → psa

But here the structure is držitel čeho? = holder of what? So the function is genitive, not accusative.

The noun držitel is a masculine animate noun.

A few clues:

• it refers to a person
• many Czech nouns ending in -tel are masculine animate, such as:
  • učitel = teacher
  • ředitel = director
  • majitel = owner
  • držitel = holder

In this sentence, držitel is the subject, so it is in the nominative singular.

Yes, very much so.

The vocabulary and structure sound official, administrative, or legal:

• držitel is formal
• ohlásit is common in official notices and regulations
• identifikační údaje is bureaucratic/administrative language

In everyday conversation, a speaker might use simpler wording, depending on the situation. But for regulations, forms, notices, or legal texts, this sentence sounds natural.

A helpful way to parse it is:

• Držitel psa = the holder/owner of the dog
• musí = must
• ohlásit = report / notify
• identifikační údaje = identification details

So the structure is:

[subject] + [modal verb] + [infinitive] + [object]

That is a very common Czech pattern:

• Student musí napsat úkol.
• Řidič musí ukázat doklady.
• Držitel psa musí ohlásit identifikační údaje.

Both can often mean to report / notify, and in many contexts they are close.

A rough guide:

• ohlásit = to announce, report, notify
• nahlásit = to report/register/provide information to an authority or system

In official usage, both may appear depending on context. In this sentence, ohlásit identifikační údaje sounds formal and natural. A similar sentence with nahlásit might also be possible in some contexts, but ohlásit has a slightly more administrative, declarative feel.

Then Czech could use the feminine noun:

• Držitelka psa musí ohlásit identifikační údaje.

So:

• držitel = male holder / generic formal holder
• držitelka = female holder

In many official texts, the masculine form is often used generically, especially in older or more traditional legal language.