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. “This” and “that” are adjectives. Sending an adjective to attempt to do the work of a noun is a kind of bad 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 reader would have had to stop reading for a while 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, supply the noun that ought to be there instead.
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!