Parallel structure is a really simple concept that you probably already know in the back of your mind, but that you probably could use a little formalism to better understand. The idea is simple: when you're building a sentence with a list of multiple entries (which may potentially be pretty much any part of the sentence), try to keep the syntactic structure of each entry the same. If you don't do this, in the best case, your sentence will be a little harder to follow, and in the worst case, it will be downright ungrammatical.
Ready for some examples? Here we go!
VN TL Example
I was mostly inspired to write this blog post by a translated script I was looking at a few months ago which had quite a lot of parallel structure problems. Here's a particularly clear example sentence demonstrating the issue, from the translated script:
She has good grades, great looks and is very popular among all the students.
Makes me want to put my fist through a wall.
The problem is that list of three items: "good grades"; "great looks"; and "is very popular among all the students." One of these things is not like the others. So, what do you do about it? You massage them until they're the same, of course. You could try to rewrite that last one to an "<adjective> <noun>" format like the others, e.g., "<some adjective> popularity", but I'm having trouble making that work. "High popularity?" Basically nonsense. Maybe chuck the word popular and find some equivalent? At any rate, I gave up on this branch because it was too much trouble already and there were better options: for starters, we could just switch to single adjectives down the line:
She's smart, pretty, and popular.
It's a little terse now, and it doesn't say exactly the same thing, but this sounds quite a lot better because of the improved parallelism. You could also go the other the direction and make them all verbal phrases:
She gets good grades, looks great, and is popular among the students.
Or even rewrite them each as a full-on independent clause:
She gets good grades, she has great looks, and she's popular among the students.
I probably wouldn't go with any of these (I actually didn't edit this line; I just left a note for the person who'd already been through it to come back and fix up the parallel structure). To be honest, most of the words here are pretty bland; the character in question is being painted in dull shades of gray. I'd want to splash some color onto it. And, yes, it should really have a serial comma (you'll see I added it to all of my versions), because the serial comma actually is as great as it's hyped up to be.
That said, neither making things more interesting (while of course keeping in mind that the goal is to better match the intent and flavor of the original) nor waxing eloquent on the value of the serial comma is my concern with this blog post. At the moment, the goal is just to avoid wanting to put my fist through a wall, and if we fix all these parallel structure problems, I might just make it through the day with my hands and walls intact.
Published News Article (if by "published" you mean "posted to some site on the internet") Example
Here's a great, more interesting example. This one I just happened to stumble across mere minutes after I'd settled on writing this blog entry earlier today as I was, out of idle curiosity, poking around for more info about that zoo penguin with a Kemono Friends waifu:
For a nation that has grown accustomed (and sometimes celebrated) an individual's obsession with their favorite waifus and husbandos, the pairing was too incredible to ignore.
My fist is twitching.
As with any writing problem, there are many ways to rewrite the sentence to fix the problem, but here's probably the least intrusive fix: insert a "to" after the "accustomed" and before the parenthetical. It's easy to convince yourself this is at least an improvement by dropping that whole additional verbal phrase inside the parentheses and reading the resulting sentence out loud to yourself, since the result is a straightforward, obviously grammatical sentence. As the sentence was originally written, if you drop the parenthetical, the result is equally obviously ungrammatical. So, here's the simple fix:
For a nation that has grown accustomed to (and sometimes celebrated) an individual's obsession with their favorite waifus and husbandos, the pairing was too incredible to ignore.
This is passable, in my opinion (assuming the audience for said news article is a bunch of weebs who will understand "waifus and husbandos"), but it's still a pretty extreme example of odd sentence structure, and if you so desire, you can go quite a bit further to fix this up for better readability, by either breaking up that list entirely or by further enhancing the parallelism. This example is particularly interesting, and the mistake here particularly understandable, because the structure of that compound verbal phrase is so complex. My initial analysis was that "to grow accustomed to" was a transitive phrasal verb, but I don't think that's quite correct, because "accustomed to XXXX" is probably best classified as an adjective, which is consistent with this usage of "to grow" being best classified as a linking verb, rather than a transitive verb. By that, I mean that "to grow a fruit tree", the transitive version of "to grow," is quite different from "to grow bored", the linking version of "to grow," of which this is an example. Meanwhile, "to celebrate" is a simple transitive verb. So in this sentence, they've actually managed to build a compound verb phrase out of: a linking verb, linking the subject to a phrasal adjective which takes an object (itself a pretty long, complex noun phrase); and the simple transitive verb "to celebrate," which is taking the same really long noun phrase for an object. Bottom line, it's still a pretty gnarly sentence because of the remaining lack of parallelism between the verb phrases.
Want to fix it more? Knock yourself out (but I think my walls are safe from me now, so please don't knock them out). Once you get parallel structure ingrained in your brain, it gets pretty easy to spot problems and to fix them. In fact, you might even start spotting correct usage of parallelism and appreciating it.