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 dont know yet when he (come) back.If the ecological situation (improve),people (feel) much better;but we still dont know when it (happen).He (not,tell) you anything if you (not,ask);but im not sure (stop) raining at all today.

I dont know when my elder brother will come back home.If he comes after eight,he will miss his favorite tv show.When Jack will return,he will help us;but i dont know yet when he will come back.If the ecological situation improves,people will feel much better;but we still dont know when it will happen.He wont tell you anything if you dont ask;but im not sure it will stop raining at all today.