Strange to say, we live in time but we don’t really know what time is. Imagine trying to define it for someone. It is probably safe to assume that most of us in our ordinary, workaday consciousness sense a persisting present moment, with a past before and a future after—past, present, future in a line of life that is always slipping off somewhere. Time, for most of us most of the time, is linear, sequential, flowing; but where it is and what it is we don’t really know.
Modern English depicts this mystery primarily in its verbs. Verbs have tense (from the Latin tempus, time), and one common and useful scheme recognizes six tenses. Three of the six are past tenses, one is the present, and two are the future. If we plot all six tenses on a timeline, it looks like this:
Past Perfect—Simple Past—Present Perfect—Simple Present—Future Perfect—Simple Future
The timeline in grammar works like the number line in math. Take the Simple Present as the present moment (zero), and everything to the left is past (negative) and everything to the right, future (positive). Taken step by step, it’s all fairly straightforward, although many of us were taught to believe that English verbs, and English grammar more largely, is a mystery beyond reckoning. It is not, and studied step by step, it unfolds into a quite subtly beautiful art.
Two of the three past tenses, the simple past and the past perfect, work together in an interesting way that is often overlooked in writing formal Standard English. In the sentence I already ate lunch when Kevin called, I have employed the simple past tense in both clauses: ate and called. It is certain, though, that my eating lunch occurred before Kevin called, because the adverb already means previously. But a quick glance at the timeline of verbs shows that in using the simple past tense ate, I have not taken advantage of another tense English has, the past perfect, to depict an action that occurred before another past action, that is to say, an action even further back in time (and so farther to the left on the timeline). Given that my eating lunch occurred in fact before Kevin’s calling, I can revise the sentence accordingly: I had already eaten lunch when Kevin called.
The simple past (ate) and the past perfect (had eaten) work complementarily like this to keep the sequence of time accurate. For this reason, the past perfect is sometimes called a relative tense, because in more formal writing (and remember, formal here means exact, not proper or prim) it works together with another past tense (expressed or implied) to state a prior action. In more relaxed contexts, though, when exactitude is less important or even necessary at all, we often settle, and appropriately so, with the original version.
Language (and some say time, too) arises from our mind, our attention to what is happening in the world. The more closely we look logically at all that is going on, the more closely we will follow the sequence of tenses in the sentences we write.