Use the present tense to describe textual action, the past tense to
describe historical circumstances.
Textual action: “In my novel, The Thirty Miles of George Washington,
Washington walks thirty miles in a single day.”
Historical circumstances: “George Washington once walked thirty miles
in a single day.”
If you need to separate time periods within textual action, use
perfect tenses (verbs with forms of “to have”).
Example: “In my novel, Washington has already completed 50 sit-ups
before he walks thirty miles in a single day.”