Flaša ulja je u kolicima.

Because ulja is the genitive singular form of ulje.

In Serbian, after a container noun like flaša (bottle), you normally use the genitive to express a bottle of ...:

• ulje = oil
• ulja = of oil

So:

• flaša ulja = a bottle of oil

This is very common in Serbian, just like:

• čaša vode = a glass of water
• šolja čaja = a cup of tea

Flaša is in the nominative singular because it is the subject of the sentence.

The sentence is built like this:

• Flaša ulja = the bottle of oil
• je = is
• u kolicima = in the cart / stroller / trolley

Since flaša is the thing being talked about, it stays in the nominative.

Also, flaša is a feminine singular noun.

Because u means in, and when it expresses location in Serbian, it takes the locative case.

So here:

• u = in
• kolicima = locative plural form of kolica

That gives:

• u kolicima = in the cart / stroller / trolley

A useful contrast:

• u kolicima = in the cart
• sa kolicima = with the cart

Also note that kolicima has the same form as the instrumental plural, so learners often mix this up. In this sentence, it is functioning as locative because it follows u with a location meaning.

Because kolica is one of those Serbian nouns that is plural in form even when it refers to one object. In grammar, this is often called a plural-only noun.

So even if English says the cart or the stroller as a singular object, Serbian uses kolica.

For example:

• Kolica su nova. = The stroller/cart is new.
• U kolicima je flaša ulja. = The bottle of oil is in the stroller/cart.

So the form is plural, even though the real-world object may be just one thing.

Kolica can mean different things depending on context, such as:

• shopping cart / trolley
• stroller / pushchair
• sometimes other wheeled carriers

So the exact meaning depends on the situation.

Without context, u kolicima could mean:

• in the shopping cart
• in the stroller
• in the trolley

Serbian often relies on context for this kind of word.

Because je is a clitic in Serbian. Clitics are short, unstressed words that usually go in the second position of the sentence or clause.

So in:

• Flaša ulja je u kolicima.

the first unit is Flaša ulja, and je comes right after it.

You can also change the word order:

• U kolicima je flaša ulja.

That is also correct, and then je still appears in second position, after U kolicima.

This second-position behavior is very typical in Serbian.

Yes. Serbian word order is fairly flexible because cases show the grammatical relationships.

These are both correct:

• Flaša ulja je u kolicima.
• U kolicima je flaša ulja.

The difference is mainly focus or emphasis:

• Flaša ulja je u kolicima. = neutral statement about where the bottle is
• U kolicima je flaša ulja. = stronger focus on the location

So the meaning stays basically the same, but the emphasis shifts.

No. Serbian does not use articles like English a and the.

So Flaša ulja can mean:

• a bottle of oil
• the bottle of oil

The context tells you which one is meant.

If Serbian wants to be more specific, it can use other words, such as:

• ta flaša ulja = that bottle of oil
• jedna flaša ulja = one / a bottle of oil

But in a basic sentence like this, there is no article.

Because u means in / inside, while na means on / on top of.

So:

• u kolicima = in the cart/stroller
• na kolicima = on the cart/stroller

If the bottle is sitting inside the basket or seat area, u kolicima is the natural choice.

A rough guide is:

• Flaša ≈ FLAH-sha
• ulja ≈ OOL-ya
• je ≈ ye
• u ≈ oo
• kolicima ≈ ko-LEE-tsee-ma

So the whole sentence is roughly:

FLAH-sha OOL-ya ye oo ko-LEE-tsee-ma

A couple of helpful notes:

• š sounds like sh
• lj is a single Serbian sound, roughly like ly in million for many English speakers

Yes, that is also possible.

Both flaša and boca can refer to a bottle, though usage can vary by region and context. In many situations they are close enough that either one works.

So:

• Flaša ulja je u kolicima.
• Boca ulja je u kolicima.

Both can be understood as The bottle of oil is in the cart/stroller.

A learner should mainly remember the grammar pattern:

• container noun + genitive
• flaša ulja
• boca ulja