I know the present tense should be used for an adverbial clause starting with "When," as in: When I become an adult, I will be a responsible person. But, I've read comments about exceptions, particularly involving "to be" in a when clause. So, is the following acceptable?
When I will be an adult, I am going to be a responsible person.
There are two somewhat different uses of when clauses with future reference.
In cases where the action of the main clause is cast in the future and you use the when clause to identify the future point more specifically, the when clause is cast in the present tense—it 'borrows' its tense reference from the main clause:
Kevin will go to London when he
will havehas the time. ... When will Kevin go to London? —When he has the time.
I am going to be a responsible person when Iwill beam an adult. ... When will you be a responsible person? —When I am an adult.
But in cases where the future point is identified and you use the when clause to provide information about something else which will happen or be in effect at that time, you may use will or BE going to in the when clause:
Kevin will go to London on Tuesday, when he will have the time. ... Why will Kevin go on Tuesday? —Because that is when he will have the time.
I am going to be a responsible person starting on my 21st birthday, when I will be an adult. ... Why will you start being responsible then? —Because I will be an adult.
This is essentially the difference between restrictive and non-restrictive relative clauses; we have many questions which address that distinction.
