Many years ago, a friend of mine had a particularly unpleasant experience on a flight to Europe. She wrote the airline a letter, and they made amends by sending her a voucher for a free flight in the future. The question for us is, why did I just use the verb wrote in the last sentence instead of has written?
Both forms, wrote and has written, are past tenses of the verb write. Both direct the reader to an action, and both place that action in a time before the present moment. The difference between the two, though, has to do with the distinction we make between an action and its consequences.
Modern English organizes our conventional experience of time (past, present, and future) into six tenses, and divides these six into two groups, three simple and three perfect. The simple tenses work like a needle piercing cloth: they pinpoint the reader’s attention simply to something that happened in the fabric of time, and they mean to say no more about it. Simple tenses, therefore, are sometimes called indefinite because they don’t intend to refer to other actions or moments of time. The verb wrote is the simple past tense.
The perfect tenses, on the other hand, refer not only to what has happened at a particular time, but to what might follow from that action as well. For this reason, the three perfect tenses in English grammar are said to be completed tenses (the word perfect means finished in all respects), because an action is thought to be complete only when its consequences, its aftereffects, are taken note of too. The perfect tenses needle the cloth, but then they pull the thread of the action forward to another point in time. Hence perfect tenses both state an action and imply its consequences. The verb has written is the present perfect tense.
So how does all this apply to my sentence? Context is critical in language, so the answer to why I chose the verb form I did has to do with the opening adverbial phrase of the sentence before it: many years ago. This short phrase set the scene far back in time, sufficiently far from the present moment that the consequences of writing to the airline—how they would respond—are now long lived out, like waves that are spent when they reach the shore. But had this anecdote occurred more recently, perhaps yesterday when the consequences cannot yet be known, then I could have chosen the present perfect: she has written the airline a letter. The recency of the event would dictate now that the consequences remain open: how will the airline respond to this letter of complaint? So the present perfect carries with it an energy of expectation that the simple past often does not.
Questions of grammar like this have an interesting way of teaching us to look more closely and more exactly at the world and how we perceive it. If it is true (and I am convinced it is) that writing and thinking are obverse and reverse of the same coin, then it behooves us to understand the forms of grammar in order to be able to communicate the nuances of a situation we are writing about.