Pas ima buvu na repu i krpelja na šapi.

Because pas is the subject of the sentence, so it stays in the nominative case.

• pas = dog / the dog
• Nominative is the basic dictionary form for the subject.

You would see psa in other roles, for example as a direct object:

• Vidim psa. = I see the dog.

But here the dog is the one that has something:

• Pas ima... = The dog has...

Because they are direct objects of the verb ima (has), so they go into the accusative case.

• buva → buvu
• krpelj → krpelja

So:

• Pas ima buvu. = The dog has a flea.
• Pas ima krpelja. = The dog has a tick.

The changes happen for different reasons:

• buva is a feminine noun, and feminine nouns in -a often change to -u in the accusative singular.
• krpelj is a masculine animate noun, and masculine animate nouns often take the same form as the genitive in the accusative singular, hence krpelja.

This is because krpelj is treated as masculine animate.

In Serbian, masculine singular nouns divide into:

• animate: people, animals, living beings
• inanimate: objects, things

For masculine singular nouns:

• inanimate accusative = same as nominative
• animate accusative = usually same as genitive

Since a tick is a living creature, Serbian uses the animate pattern:

• nominative: krpelj
• accusative: krpelja

Compare:

• Vidim sto. = I see a table.
  (sto is inanimate, so it stays the same)
• Vidim psa. = I see a dog.
  (pas is animate, so accusative changes)

Because after na when it means on in the sense of location, Serbian uses the locative case.

So:

• rep → na repu = on the tail
• šapa → na šapi = on the paw

In this sentence, the flea and tick are located on the tail and paw, so na + locative is used.

Not always. Na can take different cases depending on the meaning.

A very useful rule is:

• na + locative = location (on, at)
• na + accusative = movement toward a place/surface

Examples:

• Knjiga je na stolu. = The book is on the table.
  location → locative

• Stavljam knjigu na sto. = I am putting the book onto the table.
  movement toward → accusative

In your sentence, the flea and tick are already there, so it is location:

• na repu
• na šapi

Ima is the 3rd person singular present tense of imati (to have).

Conjugation of imati in the present tense:

• ja imam = I have
• ti imaš = you have
• on/ona/ono ima = he/she/it has
• mi imamo = we have
• vi imate = you have
• oni/one/ona imaju = they have

So:

• Pas ima... = The dog has...

Serbian has no articles, so pas can mean:

• a dog
• the dog

The exact meaning depends on context.

So Pas ima buvu... could mean:

• A dog has a flea...
• The dog has a flea...

In many learning translations, English uses the dog because it sounds more natural in a specific example sentence, but Serbian itself does not mark that distinction with an article.

Because Serbian often does not use possessive words like its, his, her when the owner is already obvious from context.

In this sentence, it is clear that the flea is on the dog’s tail and the tick is on the dog’s paw, so Serbian simply says:

• na repu = on the tail
• na šapi = on the paw

If you really wanted to stress possession, you could say something like:

• na njegovom repu = on his/its tail

But in normal Serbian, that would often sound unnecessary here.

Yes, i simply means and.

It joins the two parts:

• buvu na repu
• krpelja na šapi

So the structure is:

• Pas ima [buvu na repu] i [krpelja na šapi].

This is very natural Serbian word order.

Yes. Serbian word order is more flexible than English because cases show the grammatical roles.

The sentence as given:

• Pas ima buvu na repu i krpelja na šapi.

is the most neutral and natural way to say it.

But you could rearrange it for emphasis, for example:

• Na repu pas ima buvu, a na šapi krpelja.

That sounds more contrastive, like:

• On its tail the dog has a flea, and on its paw a tick.

So the original sentence is the best basic version, but Serbian does allow other orders when you want emphasis or contrast.

A couple of sounds may stand out to an English speaker:

• š sounds like English sh in shoe
• lj is a single Serbian sound, somewhat like the lli in million for many English speakers

So roughly:

• šapi ≈ SHAH-pee
• krpelja ≈ KRR-pe-lya

A few notes:

• The r in Serbian is trilled or tapped more than in English.
• Serbian spelling is very regular, so words are usually pronounced as they are written.

In this sentence, repeating na is natural and clear:

• na repu i krpelja na šapi

Each location phrase is its own unit:

• na repu
• na šapi

If you removed the second na, the sentence would not sound right in this structure. Serbian normally repeats the preposition when each noun has its own separate phrase.

Grammatically, it is talking about:

• one flea: buvu
• one tick: krpelja

Both are singular accusative forms.

But, just like with pas, Serbian does not use articles, so English may translate them as:

• a flea
• a tick or sometimes
• the flea
• the tick

depending on context.

The Serbian sentence itself simply tells you that the dog has a single flea on its tail and a single tick on its paw.