When explaining why you are doing something, Croatian uses two different prepositions depending on whether you are motivated by a future goal or an existing cause. Both prepositions require the genitive case.
Acting for a purpose (radi)
When you do something with a specific goal in mind, use the preposition radi (for the purpose of, for the sake of). This looks forward to a desired outcome.
Feminine nouns like kondicija (fitness) take an -e ending in the genitive (kondicije). Masculine nouns like pregled (check-up) and oporavak (recovery) take an -a ending (pregleda, oporavka).
| We have to move more for the sake of fitness. | Moramo se više kretati radi kondicije. |
| She is in the hospital for a heart check-up. | U bolnici je radi pregleda srca. |
| He stayed in bed for the sake of recovery. | Ostao je u krevetu radi oporavka. |
Acting because of a cause (zbog)
When you do something because of an existing condition or a past event, use the preposition zbog (because of, due to). This looks backward at the root cause that forces or prompts the action.
Notice how the masculine noun kašalj drops its final -a- in the genitive to become kašlja. The masculine noun tlak (blood pressure) adds -a (tlaka), and the feminine noun operacija ends in -e (operacije).
| Did he go to the check-up because of a cough? | Je li išao na pregled zbog kašlja? |
| I think that he avoids salt because of blood pressure. | Mislim da izbjegava sol zbog tlaka. |
| He couldn't work because of the heart surgery. | Nije mogao raditi zbog operacije srca. |
Comparing purpose and reason
To summarize: use radi for your goal, and zbog for the cause. Compare these two sentences about whether someone is allowed to move (kretati se):
| The doctor says that I have to move for the sake of recovery. | Liječnik kaže da se moram kretati radi oporavka. |
| I am not allowed to move a lot because of a strong cough. | Ne smijem se puno kretati zbog jakog kašlja. |