To use or not to use that

Grammar Girl has a good post on when to leave out "that". To quickly summarize, a writer can leave out that when it is not needed or when it becomes confusing such as two thats close together. But she notes that sometimes that is necessary for understanding the sentence. Consider her explanation and example:

Sometimes "That" Is Necessary

Now, there are several cases when a “that” might be necessary. If your sentence already has another “that” or two, you might not want to complicate it more by adding yet another “that” (3). One of the sentences above falls into this category. I said, “Some people think adding ‘that’ improves the flow of the sentence.” I could have also said, “Some people think THAT adding ‘that’ improves the flow of the sentence,” but I thought two cases of “that” would be a bit much.

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 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

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.

It is sometimes tricky to know if your sentence is ambiguous because you, the writer, know what you mean. I always find it useful to put aside my work for a while and then read it again with fresh eyes. You could also consider having a friend or colleague read over your work.

I find the guidelines here very helpful and clearly explained. The concern for clarity should always be primary in deciding what to write. I also like the point about getting a second opinion or putting the text aside for a while and coming back with fresh eyes.
This post on that differs from my earlier post because my post concerned a situation when that actually was a the subject of a new sentence.