forum_mail said:
Hmm... so what's the difference between future perfect and simple future ?
It is a question of completion. At the time in the future that is under discussion, will the act have been completed or not? If it has, you
should use the future perfect to express yourself more precisely, but you don't always have to do so - it depends on context. If the answer is no, you must use the simple future.
For example:
1) "If your mother is still here on Tuesday, I will have killed her."
2) "If your mother is still here on Tuesday, I will kill her."
In this case, you must use future perfect (1) if your mother-in-law will definitely
be dead by the time Tuesday arrives (assuming she has refused to leave in the time between now and then). If you are going to kill her
after Tuesday, you must use simple future.
In the case of the doubling, it is less clear. Strict linguists will tell you that you
must use the future perfect because the doubling has already occurred - it is not going to occur after the end of the year. They are right, but I would be less prescriptive. In normal usage, English speakers do use the simple future in this case ("will double"). They are technically incorrect to do so, but there is no ambiguity involved.