English utilizes verb tenses to indicate at what relative time events occurred. There are many different verb tenses, organized into categories of past, present, and future.
The past tense indicates events that took place in the past, such as in the sentence "He threw the ball."
The present tense indicates events that are taking place in the current moment, such as in the sentence "He is throwing the ball."
The future tense indicates events that will take place in the future, such as in the sentence "He will throw the ball."
Errors in verb tense occur when an author jumps back and forth between verb tenses in a way that can cause confusion to his or her reader. When writing formal literary pieces, it is preferred that authors maintain the present tense as much as possible, using the past tense only to indicate events that happened before the event being discussed at the moment.