Learn Hardcore Croatian: Učiti hrvatski je lakše kad vježbamo zajedno. - Learning Croatian is easier when we practice together.

Croatian often uses an infinitive clause as the subject, so Učiti hrvatski functions like English To learn Croatian. You can also nominalize it: Učenje hrvatskog je lakše kad vježbamo zajedno. Note the case change: with the noun učenje you need genitive hrvatskog (learning of what?).

Is je necessary, and where does it go?

Yes, you need the copula je. Croatian clitics like je usually stand in the “second position” of the clause, after the first stressed constituent. Both are natural:

Učiti hrvatski je lakše kad vježbamo zajedno.
Lakše je učiti hrvatski kad vježbamo zajedno. You cannot omit je.

Why is it hrvatski and not hrvatskog or hrvatski jezik?

hrvatski here is a substantivized adjective meaning “Croatian (language)” and is the direct object of učiti. For inanimate masculine nouns/adjectives, accusative equals nominative, so it stays hrvatski.
You can also say učiti hrvatski jezik (explicit “language”).
If you use the noun učenje, you must switch to genitive: učenje hrvatskog (jezika).

Is lakše an adjective or an adverb here?

Formally, it’s the comparative of the adjective lak. In this structure it behaves like a neuter predicative adjective agreeing with the clause-like subject (think: To je lakše = “That is easier”). Many learners also find it helpful to feel it as “more easily,” but with je the translation is “is easier.”

Why not lakši?

Lakši is the masculine comparative form. The subject here is a clause treated as neuter, so the predicative takes the neuter form lakše (parallel to To je lakše).

Can I change the word order?

Yes. Common, natural variants include:

Lakše je učiti hrvatski kad vježbamo zajedno.
Kad vježbamo zajedno, učiti hrvatski je lakše.
Hrvatski je lakše učiti kad vježbamo zajedno. Word order mainly affects emphasis, not meaning.

Should there be a comma before kad?

When the kad-clause comes after the main clause, a comma is usually omitted: … je lakše kad vježbamo zajedno. If the kad-clause comes first, you put a comma after it: Kad vježbamo zajedno, …

What’s the difference between kad and kada?

They mean the same. kada is the full form and can sound a bit more formal or careful; kad is the common shortened form. Both are standard.

Why is vježbamo in the present tense? Could I talk about the future?

Present here expresses a general/habitual truth. For a future situation you can say: Bit će lakše kad budemo vježbali zajedno. (Future in the main clause, future II in the kad-clause.)

Do I need to say mi for “we”?

No. The ending -amo in vježbamo already encodes “we.” You add mi only for emphasis: … kad mi vježbamo zajedno.

Is vježbati the right verb for “to practice” a language?

Yes. vježbati works for practicing skills, including languages. Alternatives: uvježbavati/uvježbati (to rehearse, to practice until mastered). trenirati is mostly for sports. prakticirati is “to put into practice” or “to practice (a profession),” not used for practicing a language.

Can I say učiti se hrvatski?

No. Use učiti hrvatski. The reflexive učiti se is used in different patterns (e.g., učiti se plivati can appear, but even there many prefer učiti plivati). For “learn (to completion),” use perfective naučiti: Naučiti hrvatski je lakše kad vježbamo zajedno (implies achieving the result).

What case is hrvatski here?

Accusative singular (direct object of učiti) of a substantivized adjective meaning “Croatian (language).” For inanimate masculine, nominative = accusative, so it looks like the base form.

Can I use dok instead of kad?

You can, but the nuance changes. kad = “when/whenever” (general condition or time). dok = “while” (focus on simultaneity). Učiti hrvatski je lakše dok vježbamo zajedno emphasizes “during the time that we are practicing.”

How do I pronounce tricky parts like učiti and vježbamo?

učiti: roughly “OO-chee-tee.” č like English ch in “cheek.”
vježbamo: roughly “V-yeh-zh-ba-mo.” je sounds like “ye,” ž like the s in “measure.” Stress patterns vary regionally; a safe learner approach is to keep clear vowels and the consonant sounds above.

Could I use jest instead of je?

Yes, jest is the full, stressed form of “to be” used for emphasis or in formal style: Učiti hrvatski jest lakše kad vježbamo zajedno. In everyday speech and writing, je is far more common.