Why is using 'this' mid-sentence incorrect?

Question

I am an English Literature teacher at university level, and a trained EFL teacher, so it's a bit embarrassing to have to ask this, but I can't find a satisfying answer anywhere else. My students regularly use 'this' mid-sentence, usually after a comma. E.g., 'The room is small, this indicates that...'I think it should read: 'The room is small. This indicates that...' _or_ 'The room is small, which indicates that...'I am fully aware of the rules (whether you adhere to them or otherwise) that govern the use of 'that' and 'which', but I've yet to find a convincing way to explain to my students that 'this' is incorrect mid-sentence. At the moment I try to explain that 'this' should only follow a full stop or semi-colon as it refers to the subject of the preceding clause, but that's not actually an explanation of _why_ 'this' is incorrect mid-sentence. Of course, I may be completely wrong, and it's actually fine to use 'this' in this way. Can anyone help with an explanation that I can offer my students?

Answer 1

There are two components at work here. First, using a comma when joining two complete sentences must be followed by a coordinating conjunction (for, and, no, but, or, yet, so). In order to combine two sentences without using a FANBOYS, a semi-colon must be used. Second, that is used in defining clauses; which is used for non-defining clauses.

Example: My glasses that are broken need repaired. (The pair that are broken.) -vs- My glasses, which are broken, need to be repaired. (The only pair that are owned.)