The word “that” is only very rarely optional. The bad habit of many modern English users to leave it out, even when it should be mandatory, leads to problems with ambiguity and garden-path sentences.
I have a fair bit to say on this topic, but never seem to get started. I choose to publish this miniature page (a) in the hope that I will get things done once the first step is made, (b) to at least have made the general claim somewhere.
Consider a sentence beginning with “I knew the teacher”. In a sane world, this could (only) either be a complete statement in its own right or something leading up to a statement relating to how/what/whatnot “I” knew the teacher, e.g. “I knew the teacher to be conscientious” or “I knew the teacher since 5th grade”. If the unfortunate habit of dropping “that” is allowed, however, we also have to consider possibilities like “I knew [that] the teacher would be giving a test”, which is of a very different nature and requires a re-orientation at a comparatively late point in the sentence, if there is no “that”. To avoid such issues, it must always be “I knew that the teacher would be giving a test”, never “I knew the teacher would be giving a test”.
If the original prefix is taken on the character level, not the word level, a sentence like “I knew the teacher’s wife” would also be possible. However, like the “good” examples given, and unlike the “bad” example, it does not require a drastic re-orientation. It and the “good” examples show a natural and gradual build and modification of meaning; the “bad” example gives a drastic break between apparent and actual meaning.
Note that a variation like “The teacher, I knew, would be giving a test” is of a grammatically different nature from the “bad” example and does not cause such a problem of re-orientation. (And it incidentally gives another example set through the contrast with e.g. “The teacher, [that/whom] I knew, would be giving a test”.)
A sub-issue is that language use is strongly imitative and that such artificial droppings of “that” can be infectious and/or destroy the “feel” for what is correct in others, which makes it doubly important to not continue this bad habit. (This sub-issue is generally very common with language errors, but unusually important in this case.)
The problem is an almost paradoxical counterpart to the artificial introduction of “they”—to the point that I chose the title of this page to mirror that older discussion. This could be particularly interesting with an eye at “they” and the question of what is sheer ignorance/imitation/sloppiness/whatnot and what deliberate distortion or agenda pushing, as it is hard to see any particular agenda behind removal of “that”. (However, a potentially important difference is that “that” is usually removed without replacement, while “they” is usually introduced at the cost of some other word or formulation.)
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