Kupujem pola kilograma paradajza.

Kupujem is the 1st person singular present tense form of kupovati.

So it means I buy or I am buying, depending on context. Serbian present tense often covers both:

• a general/habitual action: I buy
• an action happening now: I am buying

In this sentence, it most naturally sounds like I’m buying half a kilo of tomatoes.

No. In Serbian, subject pronouns are often omitted because the verb ending already shows who the subject is.

• kupujem = I am buying / I buy
• kupuješ = you are buying / you buy
• kupuje = he/she/it is buying / buys

So Kupujem pola kilograma paradajza is perfectly normal.

You would add ja only for emphasis or contrast, for example:

• Ja kupujem paradajz, a on kupuje papriku.
  I’m buying tomatoes, and he’s buying peppers.

In quantity expressions like this, pola is the usual word.

• pola kilograma = half a kilogram
• pola sata = half an hour
• pola hleba = half a loaf of bread

Polovina also means half, but it is used differently and sounds less natural here. It is more like one half as a noun, or one of two equal parts.

So for shopping and measurements, pola is the normal choice.

Because pola requires the following noun to be in the genitive case.

The base form is:

• kilogram = nominative

After pola, it becomes:

• pola kilograma

This is a very common pattern:

• pola sata
• pola dana
• pola litre
• pola kilograma

So kilograma is there because of the word pola.

Because after a unit of measure like kilogram, the thing being measured normally goes into the genitive as well.

So:

• kilogram paradajza
• litar mleka
• flaša vode

In this sentence, the structure is basically:

• kupujem = I am buying
• pola kilograma = half a kilogram
• paradajza = of tomatoes

So paradajza is the form required after the quantity expression.

This is a great question, because the form looks the same.

For paradajz, both:

• genitive singular = paradajza
• genitive plural = paradajza

So the form itself does not tell you which one it is.

In a sentence like this, English usually understands it as tomatoes, so learners often think of it as plural in meaning. But grammatically, quantity expressions in Serbian do not always match English one-to-one, and the important thing to notice is that the genitive form is required.

The dictionary form is kupovati, which is an imperfective verb.

That matters because Serbian verbs come in aspect pairs:

• kupovati = imperfective
• kupiti = perfective

Here, kupujem is imperfective, so it fits an ongoing or repeated action:

• Kupujem pola kilograma paradajza. = I’m buying / I buy half a kilo of tomatoes.

If you wanted a completed future action, Serbian would normally use the perfective verb:

• Kupiću pola kilograma paradajza. = I will buy half a kilo of tomatoes.

Yes. Serbian word order is more flexible than English word order.

The neutral, natural version is:

• Kupujem pola kilograma paradajza.

But other orders are possible for emphasis, for example:

• Pola kilograma paradajza kupujem.
  This puts more focus on half a kilo of tomatoes.

Even when the word order changes, the cases help show how the words relate to each other.

Yes. In everyday speech, many people would say:

• Kupujem pola kile paradajza.

Here:

• kila is a common conversational word for kilogram
• pola kile is very natural in speech

So:

• pola kilograma = standard, fully correct, a bit more formal/neutral
• pola kile = very common in everyday conversation

Both are useful to know.