According to Purdue OWL, the following paragraph is correct:
By the time Tom notices the doorbell, it has already rung three times. As usual, he has been listening to loud music on his stereo. He turns the stereo down and stands up to answer the door. An old man is standing on the steps. The man begins to speak slowly, asking for directions.
But I have the temptation to write this:
By the time Tom notices the doorbell, it has already rung three times. As usual, he has been listening to loud music on his stereo. He turns the stereo down and stands up to answer the door. An old man stood on the steps. The man begins to speak slowly, asking for directions.
Why is my version of stood incorrect? Can you recommend a good tutorial or practice at improving verb tenses?