Learn Croatian: Čak i ako kaskader odmah ne otvori padobran, ima rezervni. - Even if the stuntman doesn't open the parachute immediately, he has a spare one.

Adding čak i (even) before ako (if) turns a standard condition into a concessive conditional. It means that the outcome in the main clause (ima rezervni) remains true regardless of whether the condition (ne otvori padobran) happens or not.

Could we use pa makar instead of čak i ako here?

Yes, you could say pa makar kaskader odmah ne otvori.... Both čak i ako and pa makar mean "even if" and are largely interchangeable, though pa makar often carries a slightly stronger, more dramatic tone of defying expectations.

The verb otvori is perfective. Why is it in the present tense here? I thought perfective verbs couldn't be used in the present tense.

You're right that perfective verbs cannot be used in the present tense for independent, standalone sentences. However, in subordinate clauses like ako (if) clauses that refer to a future event, using the present tense of a perfective verb is the standard way to express that an action will be completed.

Why does the sentence end with the adjective rezervni without repeating the noun?