Why do the verb tenses change in this excerpt from The Ballad of the Sad Cafe by Carson McCullers?
"The whiskey they drink 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. "
I checked the original and your quote is not quite precise, because it begins with "The whiskey they drank that evening...." However, I do not think that is the verb shift you are asking about.
The shift from the past tense to the present tense gives this passage an immediacy it would not otherwise have. It puts the reader in the moment, drinking that whiskey and understanding its worth. The shift from the present tense to a future tense is a way of holding out for the reader the idea of what could happen while he or she is sitting there savoring the drink.
This is a beautifully written passage, with a wonderful analogy to the magic one experiences the first time one tries the lemon juice writing. All is revealed by Miss Amelia's whiskey. My favorite line, though, is "a sweetness keen as pain."
The excerpt change of verb tenses works as an argument and proof. In the present tense of the excerpt it gives you the argument, and in the past tense of the excerpt it gives you the proof. Also in the present tense it describes the effect of the liquor, and in the past tense it describes the experience of it.The excerpt starts with present tense,describing the important of Miss Amelia’s liquor and how it works once down a man’s throat. In the present tense excerpt it also describes Miss Amelia’s liquor as in the “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.” Her liquor is like the fire and the man soul is like the message. That would be the argument. Then in the past tense excerpt it states that a spinner, once drunk with Miss Amelia’s liquor, he remembers the things that have gone unnoticed, the “thoughts that have been harboured back in the dark mind, are suddenly recognized and comprehended”. That would be the proof.