Use the present simple or the future simple.
I dont know when my elder brither(come)back home.If he (come) after eight,he (miss) his favorite tv show.When Jack (return),he (help) us;but i don't know yet when he (come) back.If the ecological situation (improve),people (feel) much better;but we still don't know when it (happen).He (not,tell) you anything if you (not,ask);but im not sure (stop) raining at all today.