explain why the verb tenses change

write a paragraph (100-150 words) to explain why the verb tenses change in this excerpt.

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 liquor 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 only known in the soul of a man-- then the worth of Miss Amelia's liquor can be understood. things have gone unnoticed, thoughts that have been harboured back in the dark mind, are suddenly recognized and comprehended. A spinner who has though 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.