Verb tense can also affect the narrative style of the story. Most stories use the past tense:
I knocked on the door. She pulled out her gun.
This is usually quite adequate although flashbacks can cause awkwardness:
I had knocked on the door. She had pulled out her gun.
A little of that goes a long way.
Be careful to stay consistently in one verb tense unless your narrator is a person who might switch tenses:
So I went to see my probation officer, and she tells me I can't hang with my old buddies no more.
Some writers achieve a kind of immediacy through use of the present tense:
I knock on the door. She pulls out her gun.
We don’t feel anyone knows the outcome of events because they are occurring as we read, in “real time.” Some writers also enjoy the present tense because it seems “arty” or experimental.
But most readers of genre fiction don’t enjoy the present tense, so editors are often reluctant to let their authors use it. I learned that the hard way by using present tense in my first novel, The Empire of Time; it was enough to keep the manuscript in editorial limbo for months, and the final offer to publish was contingent on changing to past tense. Guess how long I agonized over that artistic decision!