We will have used half of all the recoverable oil that ever existed on our planet
I serached some guides about the future perfect tense and According to the sources, future perfect means the action is completed and finished at a time in the future. So, "will have used" means all of these oil will already be gone, without leaving a drop.(Please correct if I am wrong)
But Problems arise once making a tense comparison. I get confuesd when comparing it with future tense in the following sentence:
The train will have left before we reach the station.
The train will leave before we reach the station.
The first sentence means the train's departure will already finish before we are at the station, but I can't tell the future tense from future perfect tense here. Please help~
Actually it means that half the recoverable oil will have been used up but half will be left. (NB "oil" is uncountable so you can't say "these", you'd have to say "this".)
As for the train sentences, they both mean the same thing but the first one sounds better. Maybe with some verbs (other than "leave") the choice would make a difference but I can't think of one offhand. Maybe you can?