Past, Present, and Future walked into a bar...it was tense!
OK, enough foolishness. Tense is serious business when writing a story. Look at the following example:
Jack approaches the small rowboat tied to the dock. He looked out over the lake and sees the
whitecaps. The boat was pitching and rocking, so he steps directly into the center of it to keep it
Confusing? Most definitely! First Jack approaches (present tense). Then he looked (past tense).
Then he sees (present tense). The boat was pitching (past tense). Finally he steps (present tense). We don't know whether this has all happened in the past or if it's happening right now. It certainly can't be happening at both times simultaneously. Yet this confusion of tense is quite common among novice writers. The solution to this problem is to choose one tense and stick with it throughout.
It is true that the best writing process is to get your thoughts on paper ASAP so you don't forget an idea. HOWEVER, it is imperative that you go back and correct such blatant errors before submitting your story for publication. This is part of your self-editing process. Remember that no reader wants to spend time trying to figure out "what's going on when" in the middle of an action scene.
Using the example above, make the corrections necessary to create an entire story in present tense. Then go back and make it all in past tense. It's not so difficult once you realize that you need to pick one or the other--the choice is yours--and keep the action happening at the same time.
Here's another example that mixes the tenses, but in this case, it makes sense:
Jill says to Lily, "I went shopping and found some great bargains."
Lily says, "You're always lucky when it comes to saving money. I have never had that kind of
luck. I wish I could find the same kinds of buys you always seem to come across."
Believe it or not, all of the mixed-up tenses are perfectly acceptable. In this case, the actual conversation is taking place in the present, but Jill and Lily are discussing things that happened in the past, and Lily is wishing for things in the future. WHAAAAT? It is because all of the action is in the present (Jill says and Lily says), and the things they are talking about are not happening at the very same moment.
These are points you'll need to become familiar with when writing for publication. And yes, it's tense!