Many writers develop the habit of using the single word this (or that, these, those, or even it) as an all-purpose subject to point back to what I just said or to everything I’ve been talking about. The problem with using this on its own as a subject is that writers assume the reference is obvious. But far too often, the reader concludes something different or must reread a passage to figure out what the writer had in mind. The following diagram illustrates the pattern for vague "this" subjects.
To help ensure that your subjects establish necessary connections, develop a habit of always following this with a noun or descriptive phrase: this problem, this situation, this state of affairs, this antiquated approach, and so on. As you read through the following sentences and their revisions, note how vague "this" subjects make you reread or lead you to a different conclusion from that suggested in the revision.