Learn Hardcore Croatian: Piletina je bolja kad je umak malo ljut, ali ne previše slan. - Chicken is better when the sauce is a bit spicy, but not too salty.

Because it agrees with the noun piletina (chicken), which is grammatically feminine in Croatian. Predicate adjectives (adjectives after je) match the subject in gender + number + case (here: feminine singular nominative).

piletina (f. sg.) → bolja (f. sg.) (If the subject were umak (m.), you’d get bolji.)

What exactly is je doing here, and why do we see it twice?

Je is the 3rd person singular present of biti (to be). It links the subject to an adjective or noun (like English is).

Piletina je bolja = The chicken is better
kad je umak malo ljut = when the sauce is a bit spicy It appears twice because there are two clauses, and each clause has its own to be verb.

What does kad mean here, and is it the same as kada?

Kad means when. Kada is essentially the same, just a bit more formal/longer.

Piletina je bolja kad ... = common in everyday speech
Piletina je bolja kada ... = also correct, slightly more formal

Why is it kad je umak... and not something like an infinitive structure?

Croatian commonly uses a full clause with kad + a finite verb, where English might use different options (like when the sauce is... or with a spicy sauce).
Here, Croatian simply states the condition with a complete clause:

kad je umak malo ljut = when the sauce is a bit spicy

Why is umak in the nominative case?

Because umak is the subject of the subordinate clause kad je umak malo ljut. Subjects are in the nominative:

umak (subject) + je
- ljut (predicate adjective)

What is malo grammatically—an adjective or an adverb? Does it change form?

Here malo functions like an adverb meaning “a little / slightly”, modifying the adjective ljut. In this use it is invariable (it doesn’t decline).

malo ljut = a bit spicy
malo slan = a bit salty

Does ljut mean “angry”? How does it mean “spicy” here?

Yes, ljut can mean angry, but with food it very commonly means hot/spicy (like chili heat). Context decides:

On je ljut. = He’s angry.
Umak je ljut. = The sauce is spicy.

Why is it ljut (masculine) and not ljuta?

Because ljut describes umak, and umak is masculine singular. Adjectives agree with the noun they describe:

umak (m. sg.) → ljut (m. sg.) If it were juha (soup, feminine), you’d say juha je ljuta.

Why does the last part say ali ne previše slan without je?

That’s a very common ellipsis: Croatian often omits words that are easily understood from context.
Fuller versions would be:

..., ali (umak) nije previše slan. = ..., but the sauce isn’t too salty.
..., ali (umak) ne smije biti previše slan. = ..., but it mustn’t be too salty. In your sentence, (umak je) is understood, so ali ne previše slan is like but not too salty.

Why is slan masculine if we’re talking about chicken in the main clause?

Because slan still refers to umak (the sauce), not to piletina. Even though piletina is the main topic, the “not too salty” part is describing the sauce.

umak (m.) → slan (m.)

Can the word order be changed?

Yes, quite freely, as long as the structure stays clear. For example:

Kad je umak malo ljut, piletina je bolja, ali ne previše slan. (still understandable; the last adjective still points to umak) More naturally you might keep the sauce-related adjectives close together:
Piletina je bolja kad je umak malo ljut, ali ne previše slan.

Croatian has no articles—how do I know if this means “the chicken” or “chicken” in general?

You usually infer it from context. Piletina can mean:

chicken (meat) in general: a general statement about taste
the chicken (we’re eating): if you’re talking about a specific meal
If you need to be more specific, you can add words like:
ova piletina = this chicken
ta piletina = that chicken