Here is a sentence that I wrote in a message in an online discussion forum:
"I would speculate, if this statistic is really correct, that female
cyclists are more likely to be right-hooked at intersections not because
they wait for green lights but because they take a more timid lane
position, which greatly encourages, and makes a cyclist more vulneralbe
to, right-hook collisions."
Here is what I said in my mind before I started typing.
"I would speculate that, if this statistic is really correct, that female
cyclists are more likely to be right-hooked at intersections not because
they wait for green lights but because they take a more timid lane
position, which greatly encourages, and makes a cyclist more vulneralbe
to, right-hook collisions."
As I was typing, I realized that the extra "that" was probably formally incorrect but it made the sentence sound much more natural in my mind and I'm pretty sure that I use the extra "that" when I speak. So I would really like to know if other people do this and how it accords with general linguistic theory.
I'd say it's redundant here. You should be able to remove the phrase set off by commas and still have a grammatically correct phrase.
What's interesting is when you can do something like this, and I think it probably only works with hand gestures; you'd point to two women while saying this:
"The dress on the woman over there looks more expensive than that that that woman is wearing."
Of course, we can usually avoid this sort of ambiguity:
"Her dress looks more expensive than hers."
Then there's "had had"…