Ove banane su jeftine, ali ova jabuka nije jeftina.

Because ova/ove must agree with the noun in gender, number, and case.

Here both nouns are feminine, but:

• jabuka is singular → ova jabuka
• banane is plural → ove banane

The dictionary form of this demonstrative is ovaj, but it changes shape depending on the noun it goes with.

For the same reason: adjectives in Serbian also agree with the noun in gender, number, and case.

So here we get:

• jabuka = feminine singular → jeftina
• banane = feminine plural → jeftine

The basic dictionary form is usually jeftin (masculine singular), but in real sentences you often see other forms.

They are in the nominative case, because they are the subjects of the sentence:

• Ove banane su jeftine
• ova jabuka nije jeftina

The words meaning these bananas and this apple are the things being talked about, so nominative is used.

Because banane is the plural of banana.

A very common pattern in Serbian is:

• singular feminine noun in -a
• plural nominative in -e

So:

• banana = one banana
• banane = bananas

The same pattern appears in many other feminine nouns.

These are forms of the verb to be.

• su = are for they / plural subjects
• nije = is not for he/she/it / singular subject in the negative

So:

• banane is plural → su
• jabuka is singular → nije

Because the negative forms of to be are written as single words in standard Serbian:

• nisam
• nisi
• nije
• nismo
• niste
• nisu

So nije is the normal correct form for is not.

Yes. In dictionaries, adjectives are usually listed in the masculine singular nominative form, so you learn jeftin.

But in actual sentences, the adjective changes:

• jeftin = masculine singular
• jeftina = feminine singular
• jeftine = feminine plural

Since jabuka is feminine singular and banane is feminine plural, jeftin has to change.

Because here the sentence uses a predicate adjective:

• banane su jeftine
• jabuka nije jeftina

This is like English the bananas are cheap.

If you want the adjective directly before the noun, that is also possible, but the meaning shifts slightly to cheap bananas / cheap apple as a noun phrase:

• ove jeftine banane
• ova jeftina jabuka

So both patterns exist; this sentence uses the be + adjective pattern.

No, Serbian does not have articles like English the and a/an.

That means:

• jabuka can mean an apple or the apple, depending on context
• ova jabuka specifically means this apple
• ove banane specifically means these bananas

So ova/ove are not articles; they are demonstratives.

This word order is the most natural and neutral one:

• Ove banane su jeftine, ali ova jabuka nije jeftina.

Serbian word order is more flexible than English, but not every rearrangement sounds equally natural. In a basic statement, it is very common to have:

• demonstrative + noun
• verb
• adjective

So this sentence is a good standard model to follow.

Ali means but.

It introduces a contrast:

• These bananas are cheap
• but this apple is not cheap

It is one of the most common Serbian conjunctions for contrasting two statements.

Because the nouns themselves are already the subjects:

• Ove banane = the subject of the first clause
• ova jabuka = the subject of the second clause

Serbian often leaves out subject pronouns unless they are needed for emphasis. So there is no need to add a separate word for they or it here.