This is not a subject.
What kind of sentence is that? A bad one. Why? Because the subject of a sentence and the object of a preposition or verb ought to be a noun or pronoun. “This” and “that” are mostly adjectives. Sending an adjective to attempt to do the work of a noun or pronoun is a kind of bad writing.
I say they’re mostly adjectives because they can occasionally serve as articles, adverbs, or pronouns. But they’re pronouns mostly in speech.
For example, when I worked in a library, I’d answer the phone, “___ Library, this is David.”
In my very first French class, the teacher picked up items from and said their French name. Afterward, she picked them up again and asked (in French, of course), “what is this?” And we all answered, “that is a pencil” or whatever. Instead of a verbal antecedent, we had a specific object.
But what works well in speech doesn’t necessarily work in writing.
I had a professor in graduate school who used to bleed red on everyone’s papers, mostly crabbing about the writing. At least once on every paper I turned in, he wrote, “this what?” At first, I thought it was a pretty stupid question. Wasn’t it obvious from the context?
But eventually I caught on. He was trying to tell me (not very effectively!) what I am writing in this post:
I had used “this” all by itself as either a subject or an object. Whatever noun answered the question “this what?” should have been the subject or object.
“This what” may have been obvious to me, but any other careful reader would have had to stop reading for a while to figure it out.
I just came across a great illustration while I was looking for something else. Here is an extract from the web page of a company that markets an innovative way of extracting hydrogen from water:
Traditional hydrogen production is a centralized process that requires specialized and expensive infrastructure for safe storage and distribution. This constrains hydrogen’s availability and efficiency.
Systems using Hydrogen 2.0 will enable the localized production of hydrogen. This means hydrogen can be safely generated on-demand, on-site or on-board, where and when it is needed.
Making more precise writing
If “this” in the first paragraph were a pronoun, it would have to refer to an antecedent noun. We have a choice among production, process, or infrastructure. Which one is it? Any of them? I suspect the meaning is closer to “this requirement.” One red circle and a “this what” from Dr. H.
In the second paragraph, if “this” is to be taken as a pronoun, the only antecedent is “production,” because the subject of the sentence is the plural “systems.” “These mean hydrogen can . . . “ makes no sense at all. But does the writer really mean “this production”? Another circle and even more sarcastic comment, since the same infraction occurs in consecutive paragraphs.
Here’s an easy fix for the second instance: Substitute “in other words,” for “this means.”
Most readers probably sail past the slight vagueness of pressing “this” into service as a subject without noticing. Nonetheless, it’s a vagueness that careful editing ought to catch and correct. After all, careful readers will try to figure it out.
So for any of you who write, keep your reader(s) in mind, whether you are writing for all the people you hope will find your prose on the web or only for the teacher who will grade the paper.
Before you publish your writing or turn it in, read it over to look for “this” or “that.” If you find either of those adjectives pressed into service as a subject or object, substitute something more precise.
Your readers will never know how much your writing has improved as a result of your revisions, but they will be grateful. If you’re writing a paper for a class, your teacher will be grateful, even if it’s not for English class!