What does the future perfect do?

The main function of the future perfect is in future open conditionals. By a conditional, I mean a clause that includes “if” (Latin si); a future conditional is simply one that refers to a future event and its consequence; and by “open”, I mean a condition that may potentially still occur (it is "open" as to whether it will happen or not; distinguishing between “open” and “closed” conditions is a separate topic that we needn’t worry about now).Take the following future conditional in English: “If you do this, you will make a mistake”. English uses the present tense (“do”) in the first part of the conditional (known officially as the protasis) and the future tense (“will make”) in the second part (known as the apodosis). This is actually a bit strange: why use a present tense at all when you are referring to something in the future? Well, it turns out Latin thinks this, too, and it is even more scrupulous because it likes to distinguish between the future event that happens before another future event. So, the above sentence in Latin would NOT be: si hoc facis, errabis (present tense, future tense), but in fact: si hoc feceris, errabis. The verb in the first part goes in the future perfect (feceris: “you will have done”) because, if you think about it logically, you will have had to “do this” before you can be accused of “making a mistake”: the first action (denoted by the verb in the protasis) must be completed by the time the second action (denoted by the verb in the apodosis) can come about.