The basic rule is: You should use the past tense when discussing historical events, while you should use the literary present when discussing fictional events.
Literary works, paintings, films, and other artistic creations are assumed to exist in an eternal present. When you are writing about writers or artists as they express themselves in their work, stay in present tense. Here are some examples:
But when you are writing about a certain historical event (even the creation of a literary or artistic work), use the past tense. Some examples:
Sometimes a sentence must employ both present and past tense. For example:
Examine your changes of tense very carefully, however, and see if there is a logical reason for them.