Learn Croatian: Ne možemo potpisati ugovor, osim ako promijenite tu odredbu. - We cannot sign the contract, unless you change that provision.

In Croatian, perfective verbs (which describe completed actions) are almost never used in the standalone present tense. However, conditional clauses starting with ako or osim ako are a major exception. Here, the perfective present is used to express a future condition: the action of changing the provision must be completely finished for the contract to be signed.

Since this refers to a future condition, could we use the future tense and say osim ako ćete promijeniti?

No, you should avoid the future tense here. This works very much like English grammar: just as you say "unless you change" rather than "unless you will change", standard Croatian uses the present tense of a perfective verb in these "unless" clauses to express a future condition.

What is the dictionary form of tu odredbu and why does it take this ending?

The dictionary (nominative) form is odredba, which means a provision, stipulation, or clause in a legal context. It takes the -u ending because it is the direct object of the verb promijenite, which requires the accusative case. The demonstrative pronoun ta (that) also changes to its accusative form, tu.

Is the comma before osim ako mandatory?