Learn Hardcore Croatian: Ova potvrda vrijedi samo danas. - This confirmation is valid only today.

potvrda (certificate/confirmation) is feminine singular
the sentence uses nominative because it’s the subject
So you get ova potvrda (this certificate).
For comparison:
ovaj = masculine (e.g., ovaj dokument)
ovo = neuter (e.g., ovo pismo)

What case is potvrda in here, and how do I know?

potvrda is nominative singular because it’s the subject (the thing that “is valid”).
A quick test: if you turn it into a question, it answers Što vrijedi? (What is valid?) → Ova potvrda.

Why isn’t there a word for is (like je) in this sentence?

Because the main verb is vrijedi (is valid / applies / is worth). This is not a “to be + adjective” structure; it’s a normal verb sentence:

Ova potvrda vrijedi... = This certificate is valid...
You’d use je in a different structure, e.g. Ova potvrda je važeća (This certificate is valid, literally “is valid [adj.]”).

What verb is vrijedi, and how is it formed?

vrijedi is the 3rd person singular present form of the verb vrijediti.

ja vrijedim = I am worth / I count
ti vrijediš = you are worth
on/ona/ono vrijedi = he/she/it is worth / is valid
In your sentence, the subject is ova potvrda (3rd person singular), so vrijedi is the correct form.

Does vrijediti mean “to be worth” or “to be valid”? How do I know which one?

It can mean both, depending on context:

vrijediti = to be worth (money/value): Ovo vrijedi 10 eura.
vrijediti = to be valid / to apply (rules, tickets, certificates): Potvrda vrijedi do sutra.
With potvrda (certificate), the natural meaning is valid.

Is samo danas the only correct word order? Where else can samo go?

Ova potvrda vrijedi samo danas is the most neutral.
Other placements are possible, with slightly different emphasis:

Ova potvrda samo danas vrijedi. (more emphasis on today)
Samo danas ova potvrda vrijedi. (strong emphasis: “Only today…”)
Ova potvrda vrijedi danas samo. (possible, but more marked/colloquial)
In most everyday cases, keep samo directly before what it limits: danas.

Could I also say Ova potvrda vrijedi danas without samo?

Yes, but it changes the meaning.

vrijedi danas = it is valid today (doesn’t say anything about other days)
vrijedi samo danas = it is valid only today (explicit restriction)

How would I negate this sentence?

Put ne in front of the verb:

Ova potvrda ne vrijedi samo danas. = This certificate is not valid only today (i.e., it’s valid beyond today / not limited to today)
If you mean “It isn’t valid today,” you’d say:
Ova potvrda danas ne vrijedi. = This certificate is not valid today.

How do I say “valid until tomorrow” or “valid for three days” with the same verb?

Common patterns with vrijediti are:

vrijedi do + Genitive: Ova potvrda vrijedi do sutra. (valid until tomorrow)
vrijedi + time expression (often with još / ukupno): Potvrda vrijedi tri dana. (valid for three days)
You’ll also hear vrijedi do kraja mjeseca (valid until the end of the month).

How is this sentence pronounced (especially vrijedi)?

Approximate pronunciation:

Ova ≈ OH-vah
potvrda ≈ pot-VR-dah (the vr cluster is tight)
vrijedi ≈ VRYEH-dee (sounds like vri-ye- at the start for many speakers)
samo ≈ SAH-moh
danas ≈ DAH-nahs
Croatian spelling is fairly consistent: once you learn the letter sounds, reading is predictable.