Learn Hardcore Croatian: Prodavačica kaže da popust vrijedi samo danas, dok traje sniženje. - The saleswoman says the discount is valid only today, while the sale lasts.

Yes—it's nominative singular, used for the subject of the sentence (the person doing the speaking). The dictionary form is also nominative singular, so it “looks unchanged.”

Why do we use kaže da… instead of an infinitive (like “says to be valid”)?

Croatian typically uses a da-clause after verbs like reći/kažem (to say), misliti (to think), znati (to know), etc.

Prodavačica kaže da… = “The shop assistant says that…”

Using an infinitive is generally not the normal structure here in standard Croatian.

What is the function of da in the sentence?

Da introduces a subordinate content clause (a “that-clause”). Everything after da is what she is saying:

kaže da [popust vrijedi samo danas…]

Why is it popust vrijedi and not something like popust je vrijedan?

Because vrijediti is the natural verb meaning to be valid / to apply (especially for discounts, tickets, rules, offers).

Popust vrijedi = “The discount is valid/applies.”

Vrijedan means valuable / worthy (an adjective), so popust je vrijedan would mean something like “the discount is valuable,” which is not the intended meaning.

I’ve seen važi for “is valid.” Is vrijedi the same?

They overlap in meaning, but they differ by standardness and region:

vrijedi (from vrijediti) is very common and standard in Croatian for “is valid/applies.”
važi (from važiti) is also used, but many learners encounter it more in Serbian/Bosnian usage or in certain contexts. In Croatian, vrijedi is usually the safer default for “applies/is valid” in offers.

What does samo danas mean, and where can it go in the sentence?

Samo danas = only today.
It modifies the idea of how long the discount is valid.

Word order is flexible, but emphasis changes:

Popust vrijedi samo danas. (neutral: only today)
Popust samo danas vrijedi. (more emphasis on “only today,” but less neutral)
Samo danas popust vrijedi. (fronted emphasis: only today)

Why is there a comma before dok?

Because dok traje sniženje is a subordinate clause (“while the sale lasts”), and Croatian normally separates such clauses with a comma. So:

main clause: Prodavačica kaže da popust vrijedi samo danas,
subordinate clause: dok traje sniženje.

In casual writing you may see commas omitted, but the comma is standard and recommended.

Does dok mean “while” or “until” here?

Here dok means while / as long as:

dok traje sniženje = “while the sale is going on / as long as the sale lasts.”

Croatian dok can sometimes feel close to “until” depending on context, but the core meaning is “during the time that…”.

What is traje and why is it in the present tense?

Traje is 3rd person singular present of trajati = to last, to be ongoing:

(ono) traje = “it lasts / it’s ongoing”

Croatian often uses the present tense in subordinate time clauses to describe what is currently true:

dok traje sniženje = “while the sale lasts/is ongoing”

What’s the difference between popust and sniženje?

They’re related but not identical:

popust = a discount (often the specific reduction you get, e.g., “10% off”)
sniženje = a sale / markdown period (the event/period when prices are reduced)

So the sentence combines them naturally: the shop assistant says the discount is valid only today, while the sale is ongoing.

Why is it sniženje (neuter) and not something like sniženja?

Sniženje is nominative singular neuter, and it’s the subject of traje:

sniženje traje = “the sale lasts”

Sniženja could be:

genitive singular (“of the sale”), or
nominative plural (“sales/markdowns”), but neither fits the structure traje here.

How is vrijedi pronounced, and what should I watch out for in spelling?

vrijedi is roughly /vrye-di/ (three syllables: vri-je-di).
Spelling points:

ije in vrijedi is common in Croatian and represents a longer historical vowel.
Don’t drop diacritics elsewhere: Prodavačica has č, and sniženje has ž—these change pronunciation and meaning.