Kava je gorka bez šećera, ali kolač uz nju je ukusan.

Because Croatian adjectives change to match the gender and number of the noun they describe.

• kava is feminine singular, so gorak becomes gorka
• kolač is masculine singular, so ukusan stays ukusan

This is called agreement. Adjectives in Croatian must agree with the noun in gender, number, and case.

Because there are really two separate clauses, and each one has its own verb to be:

• Kava je gorka bez šećera
• ali kolač uz nju je ukusan

In English, we also repeat the verb:

• The coffee is bitter without sugar, but the cake with it is tasty

So the second je is not optional here; it belongs to the second statement.

After bez (without), Croatian uses the genitive case.

So:

• dictionary form: šećer
• after bez: šećera

This is very common and worth memorizing:

• bez mlijeka = without milk
• bez vode = without water
• bez šećera = without sugar

Because bez requires the genitive case, and the genitive singular of šećer is šećera.

So:

• šećer = nominative, the basic form
• šećera = genitive singular

This change is caused by the preposition bez.

Here uz nju means something like:

• with it
• alongside it
• served with it

In this sentence, it means the cake is tasty when eaten with the coffee or alongside the coffee.

The preposition uz often has meanings like:

• next to
• beside
• along with

So kolač uz nju is literally something like the cake next to/with it.

Because after the preposition uz, Croatian uses the accusative case.

The pronoun ona changes like this:

• ona = nominative (she/it)
• nju = accusative (her/it)

Here nju refers back to kava, which is a feminine noun. Since kava is feminine, the pronoun used for it is also feminine.

So:

• uz nju = with it / next to it

Because Croatian nouns have grammatical gender. The noun kava is feminine, so any pronoun referring back to it must also be feminine.

That does not mean coffee is biologically female. It is just a grammatical rule.

So:

• kava → feminine
• therefore nju refers to it

This is very different from modern English, where objects are usually just it.

Because uz nju is a prepositional phrase inserted after kolač to clarify which cake / the cake in relation to the coffee.

The structure is:

• kolač = cake
• uz nju = with it / alongside it
• je ukusan = is tasty

So the whole noun phrase is basically the cake with it, and then the sentence says that this cake is tasty.

Croatian often allows this kind of flexible insertion of extra information.

Yes, Croatian word order is fairly flexible. For example, you may also see:

• Kava je gorka bez šećera, ali uz nju je kolač ukusan.

This is still understandable, though the original version sounds very natural. Moving uz nju changes the emphasis a bit.

The original sentence keeps kolač uz nju together, which feels neat because uz nju closely describes the cake in this context.

Because Croatian has no articles like English a and the.

So:

• kava can mean coffee or the coffee
• kolač can mean cake or the cake

The exact meaning depends on context.

That is why learners often have to get used to reading nouns without articles and understanding definiteness from the situation.

Yes. Gorak / gorka / gorko is the usual adjective for bitter in taste.

Examples:

• gorka kava = bitter coffee
• gorka čokolada = bitter/dark chocolate
• Lijek je gorak. = The medicine is bitter.

As usual, the ending changes depending on the noun.

Yes. Ukusan is a very common word meaning:

• tasty
• delicious
• flavorful

Examples:

• ukusan kolač = a tasty cake
• Jelo je ukusno. = The food is tasty.
• Juha je ukusna. = The soup is tasty.

Again, the adjective changes to match the noun:

• masculine: ukusan
• feminine: ukusna
• neuter: ukusno

Because Croatian often puts short descriptive information after the adjective or predicate in a natural flowing way.

So:

• Kava je gorka bez šećera = The coffee is bitter without sugar

This is perfectly natural. You might also hear slightly different word orders depending on emphasis, but this one is very standard.

The phrase bez šećera explains under what condition the coffee is bitter.

Ali means but and connects two contrasting ideas:

• the coffee is bitter without sugar
• but the cake with it is tasty

So ali introduces a contrast, just like English but.

Other common Croatian connectors include:

• i = and
• a = and/but (often mild contrast)
• ali = but

Here ali is the strongest and clearest choice for contrast.

Yes, that is also natural, but the meaning shifts slightly.

• uz nju suggests alongside it / with it / served next to it
• s njom suggests with it, often in a more general sense

Both can work in context, but uz nju fits especially well if you imagine coffee and cake being served together. So the original sentence sounds very idiomatic.