What makes language so compelling a study is the insight it can give into how we perceive what is going on around us. Our human nature seems to be given over to the idea that how things seem to be is how things really are, and we are prone to forgetting that the same event can be seen from many different angles. To get our thoughts into words, we have to choose some one angle, a point of view from which we are seeing things unfold. But we can change our point of view by changing the structure of our sentences, making sentences that more accurately represent the way we experienced an event.
Take, for example, this compound sentence: I walked down the street today and I discovered my wallet was missing. We can’t really have any grammatical complaint against it: verbs agree with their subjects, there is properly no comma between the two independent clauses of like subject joined by the conjunction and, and the fact that the conjunction that is absent between discovered and my follows the common style of ellipsis, the omission of a word that can easily be assumed by the reader. But still, the sentence misfires.
The writer wants to communicate two events, walking down the street and discovering his wallet was missing, and he has depicted each event with the simple past tense: walked and discovered. The simple past (sometimes called the preterite or absolute past) is intended simply to bring attention to the entirety of a past event, to its happening and not necessarily to its connection in time to any other event, past or present. The clauses I walked and I discovered refer merely to the ideas of having walked and having discovered; they represent mental photographs of moments in time unconnected to each other or anything else. They are two separate pictures side by side in an album, the sentence, and the only way we know they do indeed have something to do with each other is by their connection through the conjunction and. But that is too much to expect from this one connective.
Instead, English has a way to change the angle from which the reader can see an action. The simple past, of course, is one of the six tenses in English, and each tense has its proper role to play. The simple past simply points to the past, but when that does not represent the event accurately or vivify the scene sufficiently, it can adjust itself subtly in three different ways while still remaining a past tense. These permutations are called aspects, and English has three of them: simple, progressive, and emphatic. The word aspect means in Latin the appearance or countenance of something, how something appears; change the aspect from simple (I walked) to progressive (I was walking), and time bursts into the sentence: I was walking down the street today. Walking is an action that can extend over time, and since the writer intends to say that something happened while walking, the progressive aspect is the better choice.
But if walking is an action that can extend over time, discovering cannot. We think of discovering something as a single act that happens at a particular point in time, not something like walking that continues, or progresses, over minutes or hours. I discovered my wallet was missing when I put my hand in my pocket at a particular culminating moment, because of which and after which an entire series of unfortunate necessities arose. And to depict this bleak realization sharply, it is best to retain the simple aspect of the verb discovered and enliven the moment with the subordinating conjunction when: when I discovered my wallet was missing. The revision, then, will have converted the writer’s original compound sentence into a complex one that is more interesting because it accords with the actuality of the scene: I was walking down the street when I discovered my wallet was missing.
We write about an objectively true and real world of events, but how we see that world, or how that world is presented to us by others, is inextricably bound up in the point of view our sentences compose for us—a fact that should brace us for the humbling realization that we might be seeing the world in just one of many possible ways.