Inappropriate Tense Shifts

Verb tense helps readers understand when something happened, when it will happen, or that it is happening.  If your tense choice is inappropriate, your readers may find the time frame in your paper confusing.  Likewise, if you go back and forth between two tenses, even though you appear to be writing about one time period, readers will be confused. 

Wallace set the museum on fire while they are both still in it.  Jarrod tried to stop Wallace, but was unsuccessful.

This excerpt from a student paper about a film starts in past tense (set), switches to present (are), and then switches back to past (tried, was) again--even though the present tense verb does not describe something that happened after the three past tense actions. 

Since this student wrote his paper after watching the film, he could have described the whole episode in past tense and avoided inappropriate tense shifts:

Wallace set the museum on fire while they were both still in it.  Jarrod tried to stop Wallace, but was unsuccessful.

However, it might be more appropriate to put the entire passage in present tense.  In academic writing, scholars often use present tense when discussing a film, a work of literature, an essay, or some other creative work:

Wallace sets the museum on fire while they are both still in it.  Jarrod tries to stop Wallace, but is unsuccessful.