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?