Future simple tense after "when"

[Saneta]

In my grammar book there is the following sentence:

You must wait for the time when you will be able to act.
Can be also:
You must wait for the time when you are able to act.? - I though it should be Present Simple after 'when'.

When should I use Simple Future tense after ,when'?

Thanks a lot for help!

[Mister Micawber]

'When' in a dependent clause normally takes the present tense for future time:
Offhand: almost always.

When I see you, I will buy you a drink.
He'll be tired when he arrives.

The 'will' form in this kind of structure is not native English unless it is indicating willpower, habit or another use of 'will', not futurity:

You'll get wet when you will go out with a raincoat.
_________________
Native English teacher at Mister Micawber's

[Saneta]

Dear Mister Micawber!
Did you want to write: ,,You'll get wet when you will go out without a raincoat.''?

[Mister Micawber]

Thanks, Saneta-- that's what I meant, indeed.
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Native English teacher at Mister Micawber's