Write in the present tense to describe the author's telling of the story. For example, "Shakespeare shows Lear's loss of life as a loss of self."
Use past tense if you are discussing the work of an author, but not what the author says in the work. For example, "Isabel Allende published "The House of Spirits" in 1982."
Combine past and present tense in the same sentence if you are referring to the historic act of creating and the events of the story in the same sentence. For example, "Written in 1984, 'The Lover' exposes illicit affairs, family drama and colonial hypocrisies in non-linear, dream-like prose."
Show a shift of time within a story by using the past or future tense. For example, "Following the sound of familiar music on the quiet ship, the girl finally realizes that she loved the man and failed to see it when it mattered."