Today in class, I discussed one of the most frequent errors I see, when grading student short stories: incorrect verb tense shifts. I think the problem is that people tend to write the way they talk. Thus, they aren't always consistent with verbs when telling a story verbally - so, it spills over into their writing quite easily.
Basically, it's all about consistency. If you start out a story in present tense, the entire story should REMAIN in present tense (except, of course, if you throw in a flashback, which would then shift to past tense verbs). The same is true for starting out a story in past tense. Everything would be past tense, not jumping back and forth between tenses. Here's a quick example (I've italicized the main verbs):
She sits at the window, looking out at the morning sky. Her decision is excruciating, and there's no way to make it less so. Feeling the weighty texture of the letter in her hands, she frowned. She remembered reading it the first time and wondered if she will have the strength to see it through.
Switching back and forth between present and past tense like that, for no apparent reason, is an error to avoid. It's funny - today, when I mentioned this frequent error in class, more than half the students nodded their heads in unison. They apparently struggle with this error, themselves.
If verb tenses are issues for you, I would suggest forgetting about them completely during the rough draft -- just let the creativity flow. The editing stage is the best time to look for these verbs, catch them, and correct them. Sentence-by-sentence, verb-by-verb. Sure, it's time-consuming and tedious. But it's also worth it. Because anything that distracts readers, that removes them from your story, should be avoided at all cost.