Sometimes "That" Is Necessary
...
Another time you should consider using a that is when your sentence could be ambiguous or misunderstood. Steven Pinker, a linguist, warns about what he calls “garden path sentences” (4). These are sentences that seem to mean one thing but then turn out to mean something else. Sometimes, keeping a that can help you avoid such problematic sentences. Pinker explains, “These are called garden path sentences, because their first words lead the listener ‘up the garden path’ to an incorrect analysis.”
Here's an example of a sentence that leads the reader down the wrong path when you omit the word that:
Aardvark maintains Squiggly's yard is too big.
Without a that, the reader is initially led to believe that Aardvark maintains, as in mows, Squiggly's yard. If you add in a that, it's clear from the beginning that Aardvark just has an opinion.
Aardvark maintains that Squiggly's yard is too big (5).
Pinker goes on to say that garden path sentences are “one of the hallmarks of bad writing” because readers have to wend their way back to the beginning of the sentence to figure out its meaning.