Parallelism is what, exactly?

If your sentence suffers from parallelism issues, it needs to have its pieces match. Think of a door with three hinges. What would happen if the top and middle hinges opened in the same way but the bottom hinge opened in the other direction? The door wouldn't function as a door at all! When you have non-parallel elements in a sentence, that sentence doesn't communicate well. It's confusing. The trick is to find what parts function as the hinges. Here is a sentence with a parallelism problem: After the maple tree clogged the downspout with seeds, rainwater had no where to go, sat for days in the gutter, and eventually the mosquitoes bred in the stagnant water. Fine, OK, we all get the picture, but the sentence seems to clunk along after we get to the word "and" because the third piece does not match the style of the first two pieces, like the bottom hinge of the poorly hung door. What are the hinges in this sentence? Rainwater had...sat...and ____. The first two hinges are verbs (actions of the rainwater) so what verb could we choose as a third hinge that would also go with rainwater and finish the sentence's last thought? Rainwater had...sat...and gave mosquitoes a perfect place to breed. Keep in mind, the hinges could be anything, not just verbs. They could be nouns, prepositions, wacky ing phrases and so on. I will provide more examples in a future post on parallelism gone wrong.