Kupujem litar ulja.

Kupujem is the 1st person singular present tense of the imperfective verb kupovati / kupujem.

It can mean both:

• I buy
• I am buying

Which one is meant depends on context. Serbian present tense often covers both the simple present and the present continuous in English.

So Kupujem litar ulja can mean:

• I’m buying a liter of oil
  or
• I buy a liter of oil (for example, habitually)

In most everyday situations, learners will often understand it as I’m buying a liter of oil.

Litar is the direct object of the sentence, so it is in the accusative case.

But litar is a masculine inanimate noun, and for many masculine inanimate nouns in Serbian, the accusative singular looks exactly the same as the nominative singular.

So:

• nominative: litar
• accusative: litar

That is why the form does not change.

Because after a word of measure like litar (liter), Serbian normally puts the following noun in the genitive.

So:

• ulje = nominative singular
• ulja = genitive singular

That gives:

• litar ulja = a liter of oil

This is very common in Serbian:

• čaša vode = a glass of water
• kilogram šećera = a kilogram of sugar
• flaša vina = a bottle of wine

Because ulje is a mass noun here, not something being counted as separate items.

You are not buying oils as multiple countable objects; you are buying oil as a substance, measured by volume. So Serbian uses:

• litar ulja = a liter of oil

The noun after a measure word is commonly in the genitive singular when it refers to an uncountable substance.

Serbian has no articles like English a/an/the.

So litar can mean:

• a liter
• the liter

depending on context.

In this sentence, English naturally translates it as a liter of oil, but Serbian does not need a separate word for a.

If you want to stress one liter, you can say:

• Kupujem jedan litar ulja.

But jedan means one, so it adds emphasis that is not always needed.

Because Serbian usually drops subject pronouns when they are not needed.

The verb ending already tells you who the subject is:

• kupujem = I buy / I am buying

So ja is unnecessary unless you want emphasis or contrast:

• Ja kupujem litar ulja, a on kupuje hleb.
  I’m buying a liter of oil, and he’s buying bread.

Without emphasis, Kupujem litar ulja is the normal way to say it.

Yes. Serbian word order is more flexible than English word order because case endings help show grammatical function.

The neutral, natural order here is:

• Kupujem litar ulja.

But other orders are possible for emphasis, for example:

• Litar ulja kupujem.
  This emphasizes a liter of oil.

Even though the word order can change, not all versions sound equally natural in all contexts. The sentence as given is the most straightforward everyday version.

The dictionary form is usually given as:

• kupovati = to buy

Its present-tense form for I is:

• (ja) kupujem = I buy / I am buying

This is an imperfective verb, which is why it is natural for ongoing or repeated actions.

A learner may also encounter the perfective partner:

• kupiti = to buy (perfective)
• kupim = I buy / I will buy, depending on context

In many beginner situations:

• kupujem focuses on the process or repeated action
• kupim focuses on completion

This is a question of aspect, which is very important in Serbian.

• kupujem = imperfective
• kupim = perfective

Very roughly:

• kupujem describes an ongoing, repeated, or habitual action
• kupim refers to a completed action

Examples:

• Kupujem litar ulja. = I’m buying / I buy a liter of oil.
• Kupiću litar ulja. = I will buy a liter of oil.
• Kupim litar ulja i vratim se kući. = I buy a liter of oil and come back home.
  (more like a completed step in a sequence)

For this sentence, kupujem is the natural choice if the focus is simply I am buying.

Normally, the standard form is Kupujem litar ulja.

The noun litar is masculine, and in standard Serbian its accusative singular is the same as nominative:

• litar

So Kupujem litar ulja is the expected form.

Learners should treat litar ulja as the standard pattern to use.

Usually you need a measure word if you are talking about a quantity of oil as a substance.

That is why Serbian naturally uses expressions like:

• litar ulja = a liter of oil
• flaša ulja = a bottle of oil
• malo ulja = a little oil

You normally would not count ulje directly the way you count separate objects like jabuke (apples). Since it is a liquid/substance, it is usually measured instead.