Should “this” ever be used as the subject of a sentence? In what types of writing is it most and least appropriate?
When I was in grad school, my adviser would cover my manuscript drafts with red ink over one error in particular: starting sentences with “This” without explaining what “This” actually refers to. Here’s an example:
Increasing the acid concentration caused the magnetization of the resultant nanoparticles to increase as well. This corroborates with our previous experimental results.
In this sentence, “This” acts as the subject. Here’s the question: This what? This behavior? This concentration? It’s not completely obvious. Now that I have learned to avoid this mistake, I see it all the time in papers I edit. When writing anything in a technical field, it is extremely important to be as specific as possible with references to other parts of the paper. Don’t assume the reader knows what you are talking about. Most often, clarifying means avoiding “this” as the subject. Change it to an adjective!
This result corroborates with our previous experimental results.
However, following this rule in other circumstances can have nasty effects. Take this excerpt from My Life and Hard Times by the humorist James Thurber:
I passed all the other courses that I took at my University, but I could never pass botany. This was because all botany students had to spend several hours a week looking through a microscope at plant cells, and I could never see through a microscope.
If I were to apply the avoid-this-as-subject rule to this sentence, what would I change it to? “This failing” or “This pattern”, perhaps? Sure, but it would look ugly and sound clunky. What does it add to the sentence? Not much. As long as what “This” references is clear ([I could never pass botany] because all botany students…), everything’s fine. If the first sentence were reversed, though, things would get messy:
I could never pass botany, even though I passed all the other courses that I took at my University. This was because…
“This” is much more unclear in this case, so “This failing” or “This pattern” would certainly clarify what it references. The obviously smarter fix is just to change the first sentence back to how it was before.
As with all of editing, blindly applying a one-size-fits-all rule is a recipe for disaster. Take into account the audience of the piece, whether clarity or style is more important, and whether there is a fix that improves both.
Image from The Thurber Carnival, copyright Harper Perennial Modern Classics