There’s a joke…
Doctor, I feel like a marquee and a wigwam.
That’s your problem, you’re two tents…
The same with your writing. Mixing past and present (and future) tense makes your readers hop about all over the place, lurching in and out of your story – ruining it for them.
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It’s not always easy to spot or to fix – sometimes it is easier to start over (NEVER delete your story, however, use it as a reference for the re-write).
Of course, if you’re more at ease using one or the other, and you find it difficult to swap, then carry on as you are. It’s YOUR story, write it YOUR way!
Personally, I prefer to write in past tense, 3rd person – a sort of omnipotent viewpoint. In my stories, the reader can see everything that’s happened and knows what the characters were thinking for the most part.
The main differences between past and present tense storytelling:
Past tense gives distance and more control over the timeline in your story, making it easy to show or hide important events relevant to the plotline or characters. Suspense and intrigue may be folded-into the story in a subtle way, leading the reader through the story on a journey of past events. The story may start out with the character at the end of the journey, so you know what happens, and the fun part of how it happens is told in the story itself.
Present tense writing is in the here and now – ‘as it happens’ reporting.
This type of storytelling is more like watching a live version of an event. Everyone is in the same boat where ‘what is about to happen’ is concerned. It can give a sense of excitement and apprehension easier than by using past-tense writing.
The trend for Young Adult novels written in present-tense could be explained in something as simple as the fashion for reality TV – watching as it happens is all around us, whether that’s on the box or in real life.
from Wiki
Crime and suspense novels are often written in past-tense format because it allows the reader to think about the action, storyline, plot and characters in a detailed, complex manner.
Children’s books are also better told in past tense because children seem to prefer stories that have happened rather than those that are happening right at that moment.
Having said all that, if you want to buck the trend and write your dystopian YA fiction in past tense, then go for it!
The only thing that is an absolute NO-NO is mixing the tenses up. Please don’t.
One example of why not is in a poem my Grandad taught me:
I went to the pictures tomorrow,
I got a front seat at the back,
They gave me a plain cake with currants in,
I ate it and gave it them back.
I went round a straight corner,
To see a dead donkey die,
I pulled out a knife to shoot it,
It went bow-legged in one eye.