When train tracks or skis are parallel, one of them doesn’t veer off. They stay the same distance apart. When writing, you want your prose to be parallel as well. When it isn’t, your readers will be jarred by the parts that veer off on their own. Here’s an example:
The most frequent causes of snowmobile accidents are mechanical failure, the driver is careless, and the weather conditions might be dangerous.
It’s easy to rewrite this sentence in parallel form, seeing that all the pieces are of the same grammatical form:
The most frequent causes of snowmobile accidents are mechanical failure, careless drivers, and dangerous weather conditions.
Here’s one more:
Roger is overworked and not paid adequately.
This is very easily fixed:
Roger is overworked and underpaid.
When you proofread what you’ve written, check to see that lists are in parallel form. If something grates on your ear, you’ll know how to fix it.