Learn Hardcore Croatian: On pokušava opet, zato uči više i govori manje svaki dan. - He is trying again, therefore he studies more and speaks less every day.

On opet pokušava (more common, neutral)
Opet on pokušava (emphatic: “Again it’s him who’s trying”) All convey “again,” but word order changes what’s emphasized.

What’s the difference between "zato" and "zato što"/"jer"?

zato = “therefore/so/that’s why” (introduces a result)
zato što and jer = “because” (introduce a reason) Your sentence uses a result: he tries again; therefore, he studies more and talks less. If you wanted the cause, you’d say:
On pokušava opet, jer svaki dan uči više i govori manje.
On pokušava opet zato što svaki dan uči više i govori manje.

Is the comma before "zato" correct? Could I use a period or semicolon?

Yes, the comma is common before zato when connecting two independent clauses. A semicolon or a period can also be used for a slightly more formal or emphatic pause:

On pokušava opet; zato uči više...
On pokušava opet. Zato uči više...

Does the present tense here mean “tries” or “is trying”?

Both. Croatian present covers English simple and continuous:

On pokušava = “He tries / He is trying”
Uči / govori = “(He) studies/is studying / speaks/is speaking”

What’s the aspectual nuance of "pokušava"? How does it differ from "pokušati" or "probati"?

pokušava is the present of the imperfective verb pokušavati (ongoing/habitual trying).
pokušati is perfective (a single, completed attempt): Pokušat će = “He will try (once).”
probati is a common synonym for “try,” often slightly more colloquial; it can also mean “to taste.” In many contexts, pokušati/probati are interchangeable.

Is "uči više" the best order, or should it be "više uči"?

Both are correct:

uči više (neutral: “studies more”)
više uči (emphasis on “more”: “he studies more (than before/others)”) Word order in Croatian is flexible and used for emphasis.

Why not "nauči" instead of "uči"?

uči (imperfective) focuses on the process of learning/studying. nauči (perfective) means “(he) will learn/ends up learning” or serves as an imperative: Nauči više! Using nauči here would change the meaning from ongoing habit to a completed result.

Is "govori" the right verb, or should it be "priča"?

Both can fit, but they differ slightly:

govori = speaks (produces speech; also “speaks a language”)
priča = talks/chats or tells stories “Talk less” in everyday advice can be govori manje or pričaj manje (imperative).

Are "više" and "manje" adjectives or adverbs here?

They function as adverbs modifying verbs (uči, govori): “study more,” “speak less.” With nouns, they quantify and take the genitive:

više vremena (more time)
manje riječi (fewer words)

Does "svaki dan" modify both verbs, or only the second one?

Placed at the end, svaki dan naturally scopes over both coordinated verbs: he studies more and speaks less every day. If you want it to modify only one verb, move it:

Only studies more daily: Svaki dan uči više i govori manje. (still likely both, but initial placement can imply routine for the whole clause) To force narrow scope, repeat it or rephrase.

Can I say "svakog dana" instead of "svaki dan"? Any difference?

Yes. svaki dan and svakog/svakoga dana both mean “every day.” svakog(a) dana (genitive) can sound a bit more formal or literary; svaki dan is very common and neutral.

Where else can "svaki dan" go in the sentence?

Several natural options:

On pokušava opet, zato svaki dan uči više i govori manje.
On svaki dan pokušava opet, zato uči više i govori manje. (emphasis on daily trying)
Svaki dan on pokušava opet, zato uči više i govori manje. (fronted for strong emphasis)

Could I start the second clause with "Zato on..." or "On zato..."?

Yes, both are grammatical and adjust focus:

Zato on uči više... (focus on “therefore he...”)
On zato uči više... (focus on “he therefore...”) Your original is neutral and natural.

Are there alternatives to "zato" for “so/therefore”?

Yes:

stoga, dakle (formal/logical connectors)
pa (colloquial “so/and so”): On pokušava opet, pa uči više...
tako da
- clause (result: “so that/as a result”): On pokušava opet, tako da svaki dan uči više i govori manje.

Is "govori manje" vs "manje govori" a meaningful difference?

Both mean “speaks less.” Default is govori manje. Fronting the adverb yields emphasis:

Manje govori = the “less” part is highlighted.