Verb tenses indicate the time when an action takes place. If you switch verb tenses, or do not use them consistently, you can end up confusing your reader by presenting an unclear timeline of events. You can certainly write about things in a different tense than they occur in reality; for instance, you can write about an event from your childhood as though it takes place right now, in the present tense, even though it happened in the past. However, you must remember to keep a consistent approach in order to avoid confusing your readers. Read on for some specific examples of correct and incorrect ways to use verb tenses.
If you switch from a past tense into a present tense, you give the reader a very specific impression that the action you describe began in the past and continues on in the present. This can create a disorienting effect, and make your reader backtrack in order to clarify the meaning of your statement.
INCORRECT: Thirty years ago, we made fun of the mouse with the iPod, but he just dances and ignores us. [Here, you see two clearly different timeframes. By switching tenses, this statement implies that the mouse has danced for 30 years.]
CORRECT: Thirty years ago, we made fun of the mouse with the iPod, but he just danced and ignored us. [This sentence remains in the past tense and clearly describes an event that happened completely in the past.]
Switching from future tense to past tense seriously disorients the reader. If you describe events in the future, make sure to use the future tense consistently. Switching into the past tense confuses your reader and creates an awkward time loop that makes no sense.
INCORRECT: The mouse will wear a blue shirt, and he danced to the Dave Matthews Band song. [Notice how this sentence begins in the future and ends in the past.]
CORRECT: The mouse plans to wear a blue shirt, and he hopes to dance to the Dave Matthews Band song.