Learn Hardcore Croatian: Oni uče hrvatski sa mnom.

Oni — subject pronoun, nominative plural, meaning they.
uče — 3rd person plural present of učiti (to learn/to study).
hrvatski — accusative masculine singular, the direct object; stands for Croatian (language).
sa mnom — prepositional phrase with me; sa
- instrumental of ja (mnom).

Why is hrvatski in the accusative, and why not hrvatskog?

Because it’s the direct object of učiti. For masculine inanimate nouns/adjectives, the accusative singular equals the nominative: hrvatski. The form hrvatskog is the accusative for masculine animate (equals genitive), which doesn’t apply to the inanimate noun jezik implied here.

Is hrvatski here an adjective used as a noun?

Yes. It’s short for hrvatski jezik (Croatian language). Croatian often drops jezik when it’s clear from context, so učiti hrvatski is perfectly natural.

Why sa mnom and not s mnom?

Both are correct; sa is used to make pronunciation easier before certain consonant clusters (like mn in mnom) and before words starting with s/š/z/ž. So sa mnom is more common than s mnom, but both are acceptable.

Why mnom and not ja? What’s the case here?

After s/sa meaning “with,” Croatian uses the instrumental case. The instrumental of ja is mnom (also mnome, formal/poetic). Mini-paradigm for reference:

Nominative: ja
Genitive/Accusative: mene
Dative/Locative: meni
Instrumental: mnom(e)

Can I drop Oni?

Yes. Croatian is a pro-drop language. Uče hrvatski sa mnom is normal when the subject is clear from context. Use Oni for emphasis or clarity.

What’s the difference between oni, one, and ona for “they”?

oni — masculine plural, also used for mixed-gender groups.
one — feminine plural.
ona — neuter plural (often for things/collectives). Choose based on the group’s gender.

Does učiti mean “to learn” or “to teach”?

Primarily “to learn, to study.” It can also mean “to teach” when taking a person as object: učiti nekoga nešto (teach someone something). To avoid ambiguity:

“I teach Croatian (as a subject)” — Predajem hrvatski.
“I teach someone Croatian” — Učim nekoga hrvatski/hrvatskom.
“I learn/study Croatian” — Učim hrvatski.

What’s the difference between učiti and naučiti?

Aspect:

učiti (imperfective) — ongoing/habitual action: “learn/study.”
naučiti (perfective) — result/completion: “learn (successfully), pick up.” Examples:
Oni uče hrvatski sa mnom. (They’re studying.)
Oni su naučili hrvatski sa mnom. (They learned it successfully.)

Is studirati a synonym of učiti?

Not generally. Studirati means “to be a university student of” (major in). Use it for formal academic programs:

Studiram pravo. (I study law at university.) For general studying/learning, use učiti.

Can I change the word order?

Yes; Croatian word order is flexible and used for emphasis:

Neutral: Oni uče hrvatski sa mnom.
Emphasize “with me”: Sa mnom uče hrvatski.
Emphasize the language: Hrvatski uče sa mnom. All are grammatical; choose based on what you want to highlight.

If I mean “from me” (I’m their teacher), should I still use sa mnom?

Use:

od mene = from me (source/teacher): Oni uče hrvatski od mene.
kod mene = at my place/with me as the instructor (location/arrangement): Oni uče hrvatski kod mene.
sa mnom = with me (together, as a study partner).

Should hrvatski be capitalized?

No. Names of languages are written in lowercase in Croatian: hrvatski, engleski, francuski. Capitalize only at the start of a sentence.

Is uče se acceptable?

You may see učiti se in the sense “be learning/be taught,” but plain učiti is preferred for “learn/study.” Use učiti se mainly in constructions like Djeca se uče čitati (Children are learning to read).

How do I negate the sentence?

Place ne before the verb:

Oni ne uče hrvatski sa mnom. You can also move elements for emphasis: Sa mnom ne uče hrvatski.

How do I make a yes/no question?

Use li after the verb, or just intonation:

Uče li oni hrvatski sa mnom?
Uče oni hrvatski sa mnom? (colloquial, rising intonation) You can also front another element for focus.

How is č pronounced in uče and hrvatski pronounced in general?

č = “ch” in “church.”
Rough guide: Oni [OH-nee], uče [OO-cheh], hrvatski [HR-vat-skee], sa mnom [sa M-nom]. The r in hrvatski is trilled/tapped.

Is sa mnom written as one word?

No. It’s always two words: sa mnom (or s mnom). The spelling samnom is incorrect.

What are the other “with X” forms with s/sa + instrumental?

sa mnom (with me)
s tobom / sa tobom (with you, singular)
s njim / s njom (with him / with her)
s nama / s vama (with us / with you plural)
s njima (with them) Using sa is common if it eases pronunciation. Also note: s/sa can take genitive for “from/off” (e.g., sa stola = from the table), but here it takes instrumental meaning “with.”