Learn Hardcore Croatian: Vježba koja je laka pomaže mom zdravlju. - The exercise that is easy helps my health.

Vježba — noun, feminine, singular, nominative; the subject (exercise).
koja — relative pronoun, feminine, singular, nominative; refers back to vježba and agrees with it.
je — auxiliary verb (3rd person singular of biti = to be); links subject and adjective in the relative clause.
laka — adjective, feminine, singular, nominative; predicate adjective describing vježba.
pomaže — verb (3rd person singular, present) of the imperfective verb pomagati; means helps.
mom — possessive adjective moj in dative singular (short form), masculine/neuter; modifies zdravlju (my).
zdravlju — noun zdravlje (health), neuter, singular, dative; indirect object (to/for my health).

Why is the relative pronoun koja used (and not koje or što)?

koja agrees with vježba (feminine singular nominative).
koje would be neuter singular or plural and wouldn’t agree.
što can function as a relative pronoun in some styles, but standard Croatian uses koji/koja/koje to refer back to a specific noun; što is more for non-nominal antecedents or colloquial use.

Do I need the verb je in the relative clause?

Yes. Croatian requires the copula in sentences like this. You can’t say Vježba koja laka; you need Vježba koja je laka.

Why is the adjective laka and not another form?

Because it must agree with vježba in gender, number, and case:

laka = feminine, singular, nominative (matching vježba).
Masculine would be lak, neuter lako.

Note: Croatian also has the variant lagan/lagana/lagano. Here, laka vježba and lagana vježba are both acceptable; laka is a bit shorter and very common.

What case is mom zdravlju, and why is it used?

It’s dative singular. The verb pomaže takes an indirect object in the dative (who/what is being helped). So we say pomaže (mojem/mom) zdravlju = helps (to) my health. English uses a direct object; Croatian uses dative without a preposition.

What’s the difference between mom and mojem in mom zdravlju?

Both mean my in the dative singular (masculine/neuter) and both are correct:

mom — short form, very common in speech and writing.
mojem/mojemu/mome — longer variants; also standard. No change in meaning here.

Is zdravlju dative or locative?

Formally, it could be either (neuter singular dative/locative share the ending -u), but here it’s dative because there is no preposition and the verb pomaže selects the dative. Locative would require a preposition like u/na/o.

Why is the verb pomaže used and not pomogne?

pomaže is present tense of the imperfective pomagati and is used for general truths/habits (Exercise that is easy helps...).
pomogne is present of the perfective pomoći and usually implies a single, bounded helping event or a potential/conditional context (e.g., Može pomoći...). For generic statements, use pomaže.

Can I express the same idea without a relative clause?

Yes:

Laka vježba pomaže mom zdravlju. (Attributive adjective instead of a relative clause.) Both are correct. The version with koja je laka slightly emphasizes the property as added information or as a restriction (the exercise that is easy, not necessarily all exercise).

Do I need commas around koja je laka?

No, not in this restrictive meaning. If you wrote Vježba, koja je laka, pomaže mom zdravlju, it would be non-restrictive (extra, incidental info: exercise, which happens to be easy, helps...), which changes the nuance.

Can I change the word order?

Yes, Croatian allows flexibility to shift emphasis:

Laka vježba pomaže mom zdravlju.
Mom zdravlju pomaže vježba koja je laka.
Pomaže mom zdravlju vježba koja je laka. The core grammar (forms and agreement) stays the same; the emphasis changes.

How do I pronounce the tricky parts?

Vježba: VYE-zh-bah (ž = zh as in vision).
koja: KO-ya (j = y).
laka: LAH-kah.
pomaže: po-MAH-zhe (ž = zh).
zdravlju: ZDRAHV-lyoo (lj = palatalized l, like Italian gli).

Could I use a prepositional alternative instead of the dative?

Yes: Laka vježba je dobra za moje zdravlje. Here za + accusative expresses “good for,” avoiding the bare dative. Your original sentence with pomaže naturally takes the dative.

What’s the difference between vježba, vježbanje, and tjelovježba?

vježba — an exercise (a specific task/activity) or exercise in general.
vježbanje — exercising/practice (the activity, gerund-like).
tjelovježba — physical exercise (PE/fitness), more formal/specific. You could say: Lagano vježbanje pomaže mom zdravlju or Laganja tjelovježba pomaže zdravlju; both are fine, with slightly different nuances.

If I want to say “easier exercise,” how do I form the comparative?

Use lakši/lakša/lakše:

Lakša vježba pomaže mom zdravlju.
Vježba koja je lakša pomaže mom zdravlju.

Can I omit the relative pronoun koja, like English sometimes omits “that”?

No. In Croatian, the relative pronoun is obligatory in such clauses. You need koja (and the verb je) to form a grammatical relative clause: Vježba koja je laka...