Kašalj je simptom prehlade.

Because prehlade is the genitive singular form of prehlada.

In this sentence, simptom is followed by a noun meaning of something:

• simptom prehlade = a symptom of a cold

So the pattern is:

• prehlada = nominative, the basic dictionary form
• prehlade = genitive singular, used here after simptom

This is very common in Serbian:

• uzrok bolesti = the cause of the illness
• simptom gripe = a symptom of the flu

The cases are:

• Kašalj — nominative singular
• je — verb, so no case
• simptom — nominative singular
• prehlade — genitive singular

So the structure is basically:

• Kašalj = subject
• je simptom = is a symptom
• prehlade = of a cold

In Serbian, after to be in sentences like this, the predicate noun is commonly in the nominative.

Because Serbian does not have articles like English a/an/the.

So:

• Kašalj je simptom prehlade.

can mean:

• Cough is a symptom of a cold
• The cough is a symptom of the cold
• A cough is a symptom of a cold

The exact meaning depends on context.

This is something English speakers often need time to get used to: Serbian usually leaves definiteness unstated unless the context makes it clear.

Je means is.

It is the 3rd person singular present tense of the verb biti = to be.

So:

• ja sam = I am
• ti si = you are
• on/ona/ono je = he/she/it is

In your sentence:

• Kašalj je simptom prehlade.
• Cough is a symptom of a cold.

In standard Serbian, you normally do include je in a sentence like this.

So:

• Kašalj je simptom prehlade. = correct

Leaving it out would sound incomplete in standard language.

This is different from some other Slavic languages where the present-tense to be can sometimes be omitted.

Kašalj is a masculine noun.

Even though it ends in -lj, it is still masculine because its basic form ends in a consonant sound.

So you would treat it as masculine in agreement:

• jak kašalj = a strong cough
• suv kašalj = a dry cough
• ovaj kašalj = this cough

Roughly, kašalj sounds like KAH-shaly.

A few helpful points:

• š sounds like English sh
• lj is a single Serbian sound, similar to the lli in million for many English speakers, though not exactly the same

So:

• ka-šalj
• approximately KAH-shaly

You do not pronounce lj as a full English l + j sequence.

Roughly:

• prehlade ≈ preh-HLAH-deh

Helpful breakdown:

• pre = like preh
• hl is pronounced clearly; Serbian often allows consonant combinations that feel unusual to English speakers
• a is like a in father
• de is deh, not dee

Serbian pronunciation is usually very phonetic, so words are generally pronounced close to how they are written.

Yes, Serbian word order is fairly flexible, though the neutral order here is:

• Kašalj je simptom prehlade.

You could also say:

• Simptom prehlade je kašalj.

That would put more focus on kašalj as the thing being identified.

So both are possible, but the original sentence is the most neutral and natural if you are simply stating a fact.

Because the sentence is making a general statement about cough as a symptom.

Serbian often uses the singular in this kind of general definition:

• Pas je životinja. = A dog is an animal.
• Kašalj je simptom prehlade. = Cough is a symptom of a cold.

English can also do this in a general way, though it sometimes prefers plural:

• Coughs are symptoms of colds
• A cough is a symptom of a cold

Serbian singular here is completely normal.

Yes. It is a very common noun + genitive pattern.

The idea is:

• noun + genitive = noun of something

Examples:

• simptom prehlade = symptom of a cold
• znak bolesti = sign of illness
• uzrok problema = cause of the problem
• lečenje gripe = treatment of the flu

So this sentence is useful not only by itself, but also as a model for many similar expressions.

Yes, you could, but it changes the nuance.

• Kašalj je simptom prehlade. = neutral, general statement
• Kašalj je jedan simptom prehlade. = cough is one symptom of a cold

Adding jedan emphasizes that it is just one among several symptoms.

In many situations, the version without jedan sounds more natural and more general.

Grammatically, it specifically says:

• prehlada = a cold

So the sentence means that cough is a symptom of a cold, not necessarily of every illness.

Of course, in real life a cough can be a symptom of many conditions, but the grammar here is specifically tied to prehlada.