Learn Hardcore Croatian: On nikad ne viče namjerno, ali ponekad slučajno zvuči preglasno. - He never shouts on purpose, but sometimes he accidentally sounds too loud.

This looks like a double negative to an English speaker, but in Croatian it is the standard way to say it. You cannot drop ne here; On nikad viče would be wrong.

Can the subject pronoun On be omitted in this sentence?

Yes. Croatian is a “pro‑drop” language, so the subject pronoun is often left out when it’s clear from context.

On nikad ne viče namjerno, ali ponekad slučajno zvuči preglasno.
Nikad ne viče namjerno, ali ponekad slučajno zvuči preglasno.

Both are correct; the second is actually more typical in natural speech and writing, unless you want to emphasize he (as opposed to someone else).

Could the word order be On nikad namjerno ne viče? Is that more natural?

Yes, On nikad namjerno ne viče is also correct and quite natural.

Core possibilities:

On nikad ne viče namjerno.
On nikad namjerno ne viče.

The meaning is the same: “He never shouts on purpose.”
The version in your sentence (nikad ne viče namjerno) is maybe slightly more neutral. Nikad namjerno ne viče puts a tiny bit more emphasis on namjerno (intentionally), but the difference is subtle. Word order with adverbs in Croatian is fairly flexible and often used to adjust emphasis or rhythm rather than basic meaning.

What’s the difference between nikad and nikada?

Nikad and nikada both mean never and are interchangeable in meaning.

On nikad ne viče.
On nikada ne viče.

Nikada sounds a bit more formal or emphatic; nikad is more colloquial and frequent in everyday speech. In this sentence you can safely use either.

What is the nuance between ponekad and slučajno here? Do we really need both?

They express two different ideas:

ponekad = sometimes, occasionally (frequency)
slučajno = accidentally, unintentionally (manner / intention)

So:

ponekad slučajno zvuči preglasno = sometimes he accidentally sounds too loud.

You could say only one:

ponekad zvuči preglasno = sometimes he sounds too loud.
slučajno zvuči preglasno = he accidentally sounds too loud (implies it happens, but doesn’t say how often).

Using both makes the sentence more precise: it happens from time to time, and when it happens, it’s not on purpose.

Can we change the order to ponekad zvuči slučajno preglasno or slučajno ponekad zvuči preglasno?

Some changes are grammatical but sound unnatural or change the nuance:

Ponekad slučajno zvuči preglasno. – Natural, standard.
Ponekad zvuči slučajno preglasno. – Grammatically OK, but sounds odd; it suggests “he sometimes sounds randomly too loud” and puts strange focus on slučajno preglasno as a chunk.
Slučajno ponekad zvuči preglasno. – Also grammatical, but slightly awkward; starting with Slučajno can be interpreted more like “By chance / incidentally, he sometimes sounds too loud”, which weakens the “not on purpose” nuance.

The original order (ponekad slučajno zvuči preglasno) is the most natural for the “sometimes, by accident” meaning.

What exactly does viče come from, and what is the difference between vikati and viknuti?

Viče is the 3rd person singular present tense of vikati (to shout, to yell – imperfective).

infinitive: vikati
he shouts: on viče

Croatian has aspect pairs:

vikati – imperfective, for ongoing or repeated action (to be shouting / to shout habitually).
viknuti – perfective, for a single, complete act (to shout once, to give a shout).

In this sentence, we’re talking about a habitual behavior (“He never shouts on purpose”), so the imperfective vikati / viče is the right choice.

Why is it zvuči preglasno and not je preglasno or preglasan je?

Zvuči is the verb zvučiti = to sound. So zvuči preglasno literally means “sounds too loud”.

On zvuči preglasno. = He sounds too loud.
On je preglasan. = He is too loud (as a general characteristic of him or his voice).

The original sentence focuses on how he comes across or is perceived (he sounds loud, not that he is loud as a permanent trait). Hence zvuči is a better fit than je here.

Why does preglasno end in -o when on is masculine? Shouldn’t it be preglasan?

Preglasno here is an adverb (too loudly), not an adjective, and it modifies the verb zvuči. In Croatian, adverbs formed from adjectives often use the neuter singular form (ending in -o or -e):

glasan (loud – masc.) → glasno (loudly)
tih (quiet – masc.) → tiho (quietly)
preglasan (too loud – masc.) → preglasno (too loudly)

So:

On je preglasan. – He is too loud. (adjective describing him)
On zvuči preglasno. – He sounds too loud / too loudly. (adverb describing how he sounds)

The masculine preglasan would be used with je when you’re describing the person or voice as a noun phrase, not how he sounds as an action.

If the person is female, how would the sentence change?

Only the pronoun changes; everything else stays the same:

Ona nikad ne viče namjerno, ali ponekad slučajno zvuči preglasno.

The verbs viče and zvuči don’t change in the present tense based on gender. The adverb preglasno also stays the same, because adverbs don’t agree in gender or number.

Why is there a comma before ali? Is a comma always required before ali?

In Croatian, ali (but) usually introduces a new clause that contrasts with the previous one, and a comma is standard before it:

On nikad ne viče namjerno, ali ponekad slučajno zvuči preglasno.

Yes, when ali joins two independent clauses (each with its own verb and subject, explicit or implied), you normally put a comma before it. Only in some very short or fixed expressions might you see it without a comma, but the safe, standard rule here is: use a comma before ali.

Could I say On nikad ne viče namjerno, ali ponekad slučajno govori preglasno? Is govori preglasno different from zvuči preglasno?

Yes, that sentence is grammatically correct:

On nikad ne viče namjerno, ali ponekad slučajno govori preglasno.

Govori preglasno = he speaks too loudly (focus on his act of speaking).
Zvuči preglasno = he sounds too loud (focus on how his voice is perceived, more general).

In everyday usage, they overlap a lot, but govori preglasno ties it more specifically to speaking, while zvuči preglasno can apply more broadly (to his tone, his voice in general, even without explicit “speaking”). Both are natural; choice depends on the nuance you want.