Rule: You must use a consistent tense in your writing. Keep to one tense system, if possible, throughout your story or essay.
Examples:
Correct (present): Brutus shows his character in all he does. He treats his wife kindly, and he mourns for her after she dies. He gladly admits that Caesar has not turned tyrant yet, but fears he may. Brutus, if he were wiser, would trust no such hypocrite as Cassius.
Correct (past): When I showed my new watch to my friends, they were impressed with what a great imitation it was. They thought it would look more like plastic, since it only cost me five dollars, but it seemed to be made of metal.
Practice using a consistent tense:
Resources for further explanation on verb tense consistency:
Web App's Verb Tense Consistency
McKendree University's Verb Tense Consistency