A lot of sentences in English are constructed with two parts that are semantically connected. We call this parallelism. Whenever you construct a sentence with parallel parts, those of us in the know consider it good form to make the parallel parts have the same structure. (Search on “parallelism” in the search box on the upper right of this page to find at least five other times wrote about this.) I remember my English teacher back in high school mentioning this, and our grammar book, Warriner’s English Grammar and Composition, had some pretty good examples, which I don’t remember. I still recommend that book if you want to have a good grammar text on hand. You can get it on Amazon. But I digress.
An example should help, because what I just wrote is rather vague. Here’s an example of a guy getting it wrong in one sentence and getting it right in the next.
To invoke another axiom, he shows rather than telling. And whether that’s a rule or a cliche, it’s true.
“Shows” is parallel to “telling,” a verb and a participle—bad. The second word should be “tells.” In the next sentence, “that’s” is parallel to “it’s,” both of which are subject-verb combinations, so that’s good. Nice even, because both are not only s-v combinations, but they’re both contractions. He’s a professional writer, a journalist even, so I suppose I should add that this rule is often broken.
But you’re better off if you don’t break it.