Consistent verb tenses clearly establish the time of the actions being described. When a passage begins in one tense and then shifts without warning and for no reason to another, readers can be distracted and confused.
Writers often encounter difficulty with verb tenses when writing about literature, as in the second example. The literary convention is to describe fictional events in the present tense.
Exercises:
Shifts: tense
All shifts 1
All shifts 2
Editing for shifts