Future Simple with if / when / after/ before / as soon as / unless / until and their synonyms or synonymous expressions. We do not use Future Tenses after these words.
Instead, we use the respective present tense.
The only exception is when. If when is used in the meaning “at what time, what period of time, what point in time,” we can use future tenses. If when means “at the time,” we can’t use any future tense, including the Future Simple.
I don’t know when he will come = I don’t know what time (or date) he will come.
When he comes, I will go = the time he comes, I will go.
Look at the rest of the examples. Pay attention to punctuation.
If you go to Brazil , I’ll go with you.
I’ll go _ if you go.
As soon as you go, I’ll go.
I’ll go as soon as you go.
Before you go, I’ll go.
I’ll go before you go.
After you go, I’ll go.
I’ll go after you go.
I won’t do it, unless they tell me to.
Unless they tell me to, I won’t do it.
Until she comes, I’ll stay.
I’ll stay, until she comes.
Recap on Future Simple with if / when etc.
When we want to use a Future Tense after:
if / when / after/ before / as soon as / unless / until,
we use one of the Present tenses instead. Except for when, if it means “what time” as apposed to “the time.” Most importantly, the second sentence normally takes the Future Simple Tense.
I will stay here until she comes.
Also on Future Simple: