Na tržnici danas kupujem lubenicu, dinju i pola kilograma marelica.

Na tržnici is the normal Croatian way to say at the market / in the marketplace.

A useful rule of thumb is:

• na is often used for places understood as public surfaces, venues, or activity spaces
• u is more often used for being inside an enclosed space

So with tržnica (marketplace), Croatian idiomatically prefers na tržnici.

Compare:

• na tržnici = at the market
• u trgovini = in the shop/store
• na placu = at the market (colloquial, in some regions)

So this is something you should mostly learn as a fixed expression.

It is in the locative singular.

The preposition na can take different cases depending on meaning:

• na + accusative = movement toward something
  
  • Idem na tržnicu. = I’m going to the market.
• na + locative = location, being at/in that place
  
  • Na tržnici kupujem... = At the market I’m buying...

Here, the sentence describes location, so tržnica becomes tržnici.

Yes, danas can move. Croatian word order is fairly flexible.

This sentence:

• Na tržnici danas kupujem lubenicu, dinju i pola kilograma marelica.

could also appear as:

• Danas na tržnici kupujem lubenicu, dinju i pola kilograma marelica.
• Danas kupujem lubenicu, dinju i pola kilograma marelica na tržnici.
• Kupujem danas na tržnici...

The basic meaning stays the same, but the emphasis changes slightly.

In your sentence, starting with Na tržnici puts the location first, and danas adds the time after that.

Croatian usually drops the subject pronoun when it is already clear from the verb ending.

• kupujem already means I buy / I am buying

So ja is not necessary.

You can say Ja kupujem if you want emphasis, for example:

• Ja kupujem lubenicu, a on kupuje dinju.
  = I’m buying the watermelon, and he’s buying the melon.

But in a neutral sentence, just kupujem is more natural.

It can mean both, depending on context.

kupujem is present tense of the imperfective verb kupovati / kupiti? More precisely, the common present-tense form is from kupovati or from the verb pair where kupiti is perfective and kupovati imperfective. In everyday usage, kupujem means:

• I am buying
• I buy
• I’m shopping for

In this sentence, because of danas and the specific shopping list, it most naturally means:

• Today I’m buying...

So English uses progressive form, but Croatian just uses the normal present tense.

Because they are direct objects, so they are in the accusative singular.

Both lubenica and dinja are feminine nouns ending in -a in the dictionary form (nominative singular). For many feminine nouns of this type:

• nominative singular -a
• accusative singular -u

So:

• lubenica → lubenicu
• dinja → dinju

This is very common in Croatian.

More examples:

• jabuka → jabuku
• knjiga → knjigu
• kava → kavu

Because nouns ending in -ja often form the accusative singular as -ju.

So:

• dinja → dinju

This follows a regular pattern for many feminine nouns of that shape.

Similar examples:

• kutija → kutiju
• linija → liniju

So dinju is exactly the expected accusative form.

This is a quantity expression, and Croatian handles these differently from English.

Break it down:

• pola = half
• kilograma = of a kilogram
• marelica = of apricots

So literally the structure is something like:

• half of a kilogram of apricots

That is why the forms are not the basic dictionary forms.

More specifically:

• kilogram becomes kilograma because it follows pola
• marelice becomes marelica because after a quantity expression like this, Croatian uses the genitive plural

So:

• pola kilograma marelica = half a kilogram of apricots

Kilograma is genitive singular.

After pola (half), Croatian normally uses the genitive:

• pola kruha = half a loaf of bread / half of the bread
• pola sata = half an hour
• pola kilograma = half a kilogram

So:

• nominative: kilogram
• genitive singular: kilograma

That is why you see pola kilograma, not pola kilogram.

Because after a measure expression like pola kilograma, the thing being measured usually goes into the genitive plural.

For marelica:

• nominative singular: marelica
• nominative plural: marelice
• genitive plural: marelica

So even though marelica looks like the singular dictionary form, here it is actually the genitive plural.

Compare:

• kilogram jabuka = a kilogram of apples
• pola kilograma marelica = half a kilogram of apricots
• dosta ljudi = enough people

This is a very important Croatian pattern: quantity expressions often trigger the genitive.

Yes. All three items belong to what is being bought:

• lubenicu
• dinju
• pola kilograma marelica

They are all part of the object of kupujem.

The first two are straightforward accusative singular nouns. The third is a quantity phrase, and inside that phrase the words follow special quantity rules:

• pola
  • genitive
• measure noun + thing measured

So the whole phrase functions as one item in the shopping list, even though internally it contains genitive forms.

This is standard list punctuation, much like in English.

The sentence lists three things:

• lubenicu
• dinju
• pola kilograma marelica

Croatian normally uses commas between earlier items in a list, but usually no comma before i (and) in a simple list:

• A, B i C

So this punctuation is completely normal.

Yes. That would also be correct.

Croatian has no articles like a or the, so a bare noun can often mean:

• a watermelon
• the watermelon
• just watermelon in context

If you want to be more specific, you can add a number:

• jednu lubenicu = one watermelon
• jednu dinju = one melon

But in many everyday situations, the bare noun is perfectly natural.

It is natural and grammatical.

A native speaker might also say some slightly different versions depending on style:

• Danas na tržnici kupujem lubenicu, dinju i pola kilograma marelica.
• Na tržnici danas kupujem lubenicu, dinju i pola kile marelica.

The last one is more colloquial because:

• kila is an everyday word for kilogram

So your original sentence is good standard Croatian, while some variants may sound a bit more conversational.

Yes, but they differ in style and region.

• tržnica = standard word for market / marketplace
• plac = common colloquial word in many areas, especially for an open-air produce market

So:

• Na tržnici kupujem... = standard
• Na placu kupujem... = more informal/regional

Both are useful to know, but tržnica is the safer standard choice for learners.