On Fixing Sentences, a lot gets made out of the intrinsic wrongness of the word being. Yes, it’s awful, yes it’s dangerous, yes it’s wrong 98% of the time. But it’s not the only dangerous word on Fixing Sentences. In fact, I would argue that along with it, which is also wrong a very high percentage of the time, this is probably the next most dangerous word on the Writing section, particularly Fixing sentences. And it really shows up a lot.
If you’re looking for a very general rule, here goes: When this is immediately followed by a noun (e.g. this book, this fact, this idea), it’s right; when there’s no noun, it’s wrong. Usually there won’t be a noun.
Now for the explanation. Like it, this is a singular pronoun, which means that it must refer to a specific singular noun (or pronoun or gerund) that appears in the sentence. If the specific noun to which this refers (the antecedent) does not appear, the sentence is incorrect. For example:
Incorrect: Members of the local government have requested that more traffic lights be installed throughout the city because they believe that this will help to prevent accidents.
What does this refer to? it could refer to the traffic lights, but that’s plural. It could also refer to the installation of the traffic lights, but installation is a noun, and only installed, a verb, appears in the sentence. So there are a couple of ways to fix the sentence. You could provide a noun that clearly specifies what is being referred to:
Correct: Members of the local government have requested the installation of more traffic lights throughout the city because they believe that the lights will help to prevent accidents.
OR you can simply add a noun after this and specify what it refers to.
Correct: Members of the local government has requested the installation of more traffic lights throughout the city because they believe that this addition will help to prevent accidents.
Now, onto that:
Unlike this, that is usually correct when it shows up in a sentence. When it’s underlined in Error-IDs, it’s usually used this way:
Correct: The local believes that installing more traffic lights throughout the city will help to prevent accidents.
Correct: The additional traffic lights that have recently been installed throughout the city are expected to help prevent accidents.
In the latter case, you might wondering why that rather than which should be used. The short answer is that which is always preceded by a comma and used to set off a non-essential clause (e.g. The additional traffic lights, which have recently been installed throughout the city, are expected to help prevent accidents.) That, on the other hand, is never preceded by a comma.