Why do the verb tenses change in this excerpt?
"The whiskey they drank that evening (two bottles of it) is important. Otherwise, it would be hard to account for what followed. Perhaps without it there would never have been a cafe. For the liquour of Miss Amelia has a quality of its own. It is clean and sharp on the tongue, but once down a man it glows inside him for a long time afterward. And that is not all. It is known that if a message is written with lemon juice on a clear sheet of paper there will be no sign of it. But if the paper is held for a moment to the fire then the letters turn brown and the meaning becomes clear. Imagine that the whiskey is the fire and that the message is that which is known only in the soul of of a man--then the worth of Miss Amelia's liquor can be understood. Things that have gone unnoticed, thoughts that have been harboured far back in the dark mind, are suddenly recognized and comprehended. A spinner who has thought only of the loom, the dinner pail, the bed, and the loom again--this spinner might drink some on a Sunday and come across a marsh lily. And in his palm he might hold this flower, and in him might come a sweetness keen as pain."
Some of the verbs in this excerpt are in the present tense, some are in passive voice, some in future construction, and some in present perfect tense. Let us briefly discuss some of this, talk a little bit about conditionals, and then go into the excerpt.
SIMPLE PRESENT TENSE:
Aside from its primary use of relating the NOW, the simple present is also used "in the narration of events set in the past" (American Heritage Dictionary). The use of the simple present in these moments add drama or vividness to a narrative. In this usage, the simple present is called the historical present, the dramatic present, or the narrative present.
PRESENT PERFECT TENSE:
The present perfect tense is used to express events that have happened at an indefinite time in the past but that are connected to the present through your thoughts or conversaton. These actions may have happened only once or persisted for a while.
There are four types of conditionals: zero conditional, first conditional, second conditional, and the third conditional. Among these, the second conditional is used to describe the future and the third conditional is used to describe events that are contrary to past facts, things that did not happen. Their forms are usually as follows (second, then third):
if + simple past, would/might/should/could + base verb
if + past perfect, would have + past participle
As you may well have guessed, the excerpt narrates a pivotal moment that ushered in the establishment of a cafe. Hence, the historical present. The third sentence in the excerpt illustrates a variation of the third conditional. "Perhaps without it there would never have been a cafe." Perhaps there would never have been a cafe had there been no whiskey. But there was. The succeeding sentences are written using the simple present to express some facts/generalizations until the part when the narrator tells of "things that have gone unnoticed... have been harboured far back in the dark mind..." The reason why the verbs change between tenses is mostly that with the present perfect tense, the narrator is able to signify the time of occurrence of these events - at some unspecified time before the historical present.