Like a lot of languages (but not all), English is set up to indicate the time of an action—past, present, or future—by changing the way its verbs are spelled and arranged. We all (at least we all in the modern world) seem to have a sense that things happen in a sequence, that one thing follows the next: I ate lunch yesterday just before noon, and today I will probably eat around 12:30. Time seems to be bound up with the way we experience the world, so if we intend to communicate our experiences to someone through language, we have to be clear about what happened when.
This is particularly important in sentences that project more than one idea into the future. Let’s say you want to invite me for coffee and you say, Would tomorrow about 1:30 be OK? Maybe we can get some lunch, too. You are casting an idea over a stretch of time between the moment we are speaking and another moment called tomorrow about 1:30; and although the present keeps moving inexorably toward that future tomorrow about 1:30 moment, there’s enough space-time, so to speak, for other things to happen. So if I then reply, Coffee’s great, but I will have eaten lunch by then, I have inserted the idea of eating lunch between the present moment (the moment we are talking) and the future moment of tomorrow about 1:30. My eating lunch is in the future—from the perspective of the present, the moment we are talking, but it is in the past too—from the perspective of tomorrow about 1:30. Curious things, our mind and its language.
To represent this state of affairs, English has a tense called the future perfect. The term perfect in grammar means past or completed or finished, so on the face of it, the name of the future perfect tense appears to be a self-contradiction: how can something be both in the future and already past? The answer, of course, lies in the fact that our mind can assume any position it likes along the line of time. If we stay where we are in the very real present, then coffee tomorrow about 1:30 is certainly a future moment. But if in our imagination we fly up and out to that very moment and assume life is going on there, then my eating lunch, let’s say at noon, will certainly appear to be something that happened in the past. Noon is future to the present moment and past to a future moment. And that’s why the future perfect tense is not a contradiction in terms.
So now, if we look closely at how the future perfect tense is constructed, much of the mystery evaporates. The tense is expressed in a phrase (called, appropriately enough, a verb phrase), which means it needs more than one verb to communicate its meaning. In a verb phrase, the last verb is always the one that carries the idea (it’s called the principal or notional verb), and any other verbs before it specify, among other ideas, the tense; they help the principal verb and are called auxiliary verbs. So in our verb phrase will have eaten, eaten is the principal verb and will and have are two auxiliary verbs; will is there to indicate the future, and have to indicate the perfect, or past. Thus the future perfect tense.
It’s really quite an ingenious little device—both a sharp mental instrument to slice up time and a sleight of hand to make us believe we can ever live elsewhere but in the present. But that seems to be in the essence of language, sending us here and there, past and future, building a mental world that we superimpose on the living and actual one, the present one, the only one we are ever really in.