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Are there rules about using “that” to join two clauses?
If a sentence makes sense to me without the word "that", I tend not to use it. However, I have recently come across a number of "thats" in colleagues work where I wouldn't use them. When should it be used?
You can use or omit that however you want, until you break any grammatical rules. Both of these are correct:
Do you think John will be late?
and
Do you think that John will be late?
This can hardly be called overusing the word that. In some cases using the word can help disambiguate the sentence or at least make it a lot easier to parse. I cannot think of any concrete examples right now, but I'll be thinking about it in the next hour and I'll come back and edit my answer to add the example when one comes into my mind.
