Learn Hardcore Croatian: Sport im je važan, jer kroz sport upoznaju nove prijatelje. - Sport is important to them because through sport they meet new friends.

It’s the unstressed dative pronoun im = “to them” (3rd person plural). Croatian says “X is important to someone” with the dative: Nešto je (nekome) važno. So Sport im je važan literally means “Sport is important to them.”

Why is the order im je, and why does it appear right after Sport?

Croatian clitics (unstressed short words) like im and je go to second position in the clause, right after the first stressed word or phrase. Here the first stressed word is Sport, so the clitic cluster im je comes next. Within that cluster, pronoun clitics normally precede je, hence im je, not je im. You can also say:

Važan im je sport.
Njima je sport važan. (full form njima adds emphasis)

Why is it je and not su?

Because the subject Sport is singular. je = 3rd person singular of biti (to be). su is plural.

Why is the adjective važan (masc. sg.) and not važno or važna?

Adjectives agree with the subject. sport is masculine singular, so važan. If the subject were feminine (škola), you’d say važna; if neuter (učenje), važno.

Could I use bitan instead of važan?

Yes. bitan is a common synonym. Sport im je bitan is fine.

Why is there a comma before jer?

Croatian uses a comma before jer (“because”) when it introduces a reason clause: …, jer …. That comma is standard.

Is there any difference between jer and zato što?

Both mean “because.” jer typically follows the main clause: Sport im je važan, jer …. When the reason comes first, learners often prefer zato što: Važan im je sport zato što kroz sport upoznaju nove prijatelje. Starting a sentence with Jer is possible in speech but is less common in formal writing.

Why is it kroz sport? Which case does kroz take?

kroz (“through”) governs the accusative. sport is masculine inanimate; its accusative singular looks the same as the nominative: kroz sport.

Could I say u sportu instead of kroz sport?

Yes, but there’s a nuance:

kroz sport = “through/by means of sport” (sport as a channel/means).
u sportu = “in sport(s)” (within the domain of sport). Here kroz sport fits the idea of sport as the medium for meeting people.

What exactly is upoznaju? Which verb and aspect is it?

It’s 3rd person plural present. In this context it’s the present of the imperfective upoznavati (“to meet/get to know” habitually): upoznajem, upoznaješ, …, upoznaju. For a single, one-off meeting you’d typically use the perfective upoznati in the future: upoznat će (nekoga).

Do I need the reflexive se with this verb?

Two patterns are common:

upoznati/upoznavati + Acc. = “meet someone”: upoznaju nove prijatelje.
upoznati se/upoznavati se s/sa + Instr. = “get acquainted (with someone)”: upoznaju se s novim prijateljima. Here the non‑reflexive form is the right choice.

Why is it nove prijatelje, not novih prijatelja?

Because upoznaju takes a direct object in the accusative. Masculine animate plural accusative is -e: prijatelje, and the adjective agrees: nove prijatelje. novih prijatelja is genitive plural (used with quantities, certain prepositions, or negation), not appropriate here.

What case is prijatelje, and why not prijatelji?

prijatelje is accusative plural (direct object). prijatelji is nominative plural (subject). Compare:

Prijatelji dolaze. (subject)
Upoznajem prijatelje. (object)

Can I avoid repeating sport in the second clause?

Yes. You can use a pronoun: …, jer kroz njega upoznaju nove prijatelje. (njega = “it,” referring to masculine sport). Just make sure context doesn’t make njega sound like “him.”

Is the word order in the second clause flexible?

Yes. Adverbials like kroz sport move freely:

…, jer kroz sport upoznaju nove prijatelje.
…, jer upoznaju nove prijatelje kroz sport.
…, jer upoznaju kroz sport nove prijatelje. (slightly marked focus)

Where do clitics go if I change the order of the first clause?

They still go to second position in their clause:

Važan im je sport, … (starts with an adjective; clitics follow it)
Njima je sport važan, … (full njima isn’t a clitic; je is now the clitic in second position)
With a fronted reason: Zato što kroz sport upoznaju nove prijatelje, sport im je važan. (im je still follows Sport)

Could I use the plural sportovi here?

You could, but it changes the meaning. Sport im je važan treats “sport” as an activity in general. Sportovi su im važni means “sports (various kinds) are important to them,” focusing on multiple sports rather than the concept of sport.