while in first sentence we do not use future but second USE FUTURE simple?????? Thank
while in first sentence we do not use future but second USE FUTURE simple?????? Thank
Could you eplain in simple way in order learner of English undrstood
while in first sentence we do not use future but second USE FUTURE simple?????? Thank
He will explain everything when he comes.
I don't know when he will come.

Question

while in first sentence we do not use future but second USE FUTURE simple?????? Thank
while in first sentence we do not use future but second USE FUTURE simple?????? Thank 
Could you eplain in simple way in order learner of English undrstood 
while in first sentence we do not use future but second USE FUTURE simple?????? Thank 
He will explain everything when he comes. 
I don't know when he will come.

Answer

"He will explain everything when he comes."  In this sentence, "when he comes" is a time.  We keep time-clauses in simple tense because the main part of the sentence needs the future tense. If we add "will" to the time-clause, not only is it technically wrong, but it's also confusing to the listener/reader. We wouldn't know which part of the sentence is the main clause.

"I don't know when he will come." In this sentence, "when he will come" is the object of the sentence. It's the thing that you don't know. In this case, you can use will.