Psa bolí tlapka.

Because pes changes form here.

With the verb bolet in this kind of sentence, the person or animal that feels the pain is usually in the accusative:

• pes = nominative singular
• psa = accusative singular

So Psa bolí tlapka uses psa because the dog is the one experiencing the pain.

A very literal gloss is:

• psa = the dog
• bolí = hurts
• tlapka = paw

So structurally it is closer to The paw hurts the dog than to normal English wording.

In this sentence, psa is accusative singular.

It is true that psa can also be the genitive singular form of pes, so the form by itself is ambiguous. But in Psa bolí tlapka, the grammar of the sentence shows that it is accusative:

• tlapka is nominative
• psa is the affected animal, so it is accusative

So here it definitely means the dog as the experiencer of pain, not a genitive meaning like of the dog.

The grammatical subject is tlapka.

That may feel strange to an English speaker, because English usually says something like The dog’s paw hurts, where dog’s paw is one unit. But Czech often builds pain sentences differently:

• Mě bolí hlava = My head hurts / literally The head hurts me
• Psa bolí tlapka = The dog’s paw hurts / literally The paw hurts the dog

So:

• tlapka = subject, nominative
• psa = object-like experiencer, accusative

Because Czech often leaves possession unstated when it is obvious from context, especially with body parts.

So Psa bolí tlapka is naturally understood as:

• The dog’s paw hurts
• not just any random paw

This is very common in Czech. Compare:

• Bolí mě hlava = My head hurts
• not usually Moje hlava mě bolí

You can add a possessive if needed for emphasis or contrast, but it is usually unnecessary.

Because the verb bolet normally uses the accusative for the one who feels pain.

So the standard pattern is:

• Bolí mě zub = My tooth hurts
• Bolí ho noha = His leg hurts
• Psa bolí tlapka = The dog’s paw hurts

An English speaker might expect something like to the dog, but Czech does not use the dative here. For this verb and this meaning, the normal case is accusative.

Because Czech word order is flexible, and it often reflects topic and focus rather than strict subject-verb-object order.

Starting with psa makes the dog the topic, as if you are talking about the dog and then saying what is wrong with it.

So:

• Psa bolí tlapka = natural if the dog is already what you are talking about
• Tlapka bolí psa = grammatical, but less natural in many contexts and more focused on tlapka

For learners, the main point is:

• Czech endings show the roles
• word order often shows emphasis or information flow

Czech has no articles, so the sentence itself does not explicitly mark the versus a.

So psa can mean either:

• the dog
• a dog

Context tells you which one is meant.

In an isolated teaching example, English often uses the dog, because it sounds more natural when talking about a specific body part.

You would usually say:

• Psa bolí tlapky.

Here only tlapka changes to plural:

• tlapka = paw
• tlapky = paws

The verb stays bolí, because that is the same form used here as well.

So:

• Psa bolí tlapka = The dog’s paw hurts
• Psa bolí tlapky = The dog’s paws hurt

Yes. A very common alternative is:

• Pes má bolavou tlapku.

That means something like The dog has a sore paw.

The difference is roughly:

• Psa bolí tlapka = focuses on the pain itself
• Pes má bolavou tlapku = describes the paw as sore/injured

Both are natural, but they are not exactly the same in nuance. The original sentence is the more typical pain construction with bolet.