Hi Egor, if the MAIN verb in the sentence is PRESENT, like in your example (I'M NOT SURE), using WILL (FUTURE) after WHEN (in the subordinate clause) is perfectly correct. But in the following example you have to use the PRESENT after WHEN: "I WILL be happy WHEN she ARRIVES". I mean, when the MAIN verb in the sentence is FUTURE, you use the PRESENT after WHEN (in the subordinate clause).