| If the time adverbial clause referes to somthing that will happen or exists in the future, you use the simple present tense, not a future tense. |
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You have stated a very important rule here:
Look before you leap. (not
before you will leap)
He is going to the bank before he has dinner. (not
before he will have dinner)
The Joneses will buy a new car before they take their vacation. (not
before they will take)
Will you set the alarm before the children go to bed? (not
will go to bed)
But the rule (obviously) has some exceptions. I cannot claim to
have investigated every possible combination and permutation possible,
but it seems to me that the prerequisites for breaking this rule are
1) the agents of the two clauses must be different
2) there must be some reaction involved, the subordinate clause showing the reaction to the action of the main clause
3) the main clause involves of modal of obligation in the present tense
The model is:
You have to do X before I will do Y (or
before Y will be done).
The children [must/have to] say "Please" before Auntie Em will give them candy.
We [must/has to] pay the station attendant before gas will come out of the pump.
The Smiths [must/have to] apologize before their neighbors will speak to them again.
[Do you have to / Must you] tell the children a story before they will go to bed?
These examples are a matter of speculation. I invite the other
members of the forum to add to them or to find counterexamples or to
revise the set of three conditions which I suggested as an explanation
for such exceptions to the basic rule. Note that these exceptions
always (I think) make the "will" sound like a verb of
willingness. The desired effect will not take place until the
required act is completed. The required act must occur before
some other agent will (is willing to) fulfill his part.
I also think that all three examples I cited can also be stated in the simple present (
... Auntie Em gives them ..., etc.)
CJ