Sometimes you need them, sometimes you don't.
Syntaxis workshop participants often have questions about the word that. One common question is whether that can be omitted from certain sentences, as in:
She said he was a hard worker.
She said that he was a hard worker.
The answer is yes. If the meaning of a sentence without the that is unambiguous, and if you prefer the way the sentence sounds without it, delete away.
Here are several other acceptable sentences with an implied rather than explicit that:
Susan proved she could do it.
Mark and Barbara claimed they would be able to complete the project by 5:00 p.m.
The presentation they gave last night was fascinating.
In some cases, deleting an unnecessary that can help reduce repetition. You have probably encountered sentences with multiple instances of that in quick succession, as in:
He said that the report that Janet was writing would show that our costs had been spiraling out of control.
Instead of writing that three times in this (currently inelegant) sentence, delete a couple of unnecessary ones. Besides reducing repetition, the deletions will also make the sentence sound less convoluted:
He said the report Janet was writing would show that our costs had been spiraling out of control.
Don’t be reckless with the that deletions, though. In some cases, omitting that can create temporary confusion about a sentence’s structure and meaning. For example:
He believed John was a liar.
This sentence is not incorrect, but as you read the first three words, it seems at first as though John is the direct object of the verb believed (as in He believed John). In business writing, where clarity is a top priority, it is often a good idea to preserve the that in sentences with this type of structural ambiguity. Although the structure in our example above will become clear as the reader continues past the word John, including the word that makes the writer’s meaning apparent right away:
He believed that John was a liar.
Below are a few other examples, structurally similar to the one above, where keeping the word that enhances clarity:
She saw that Marshall was struggling. [to avoid the misreading She saw Marshall…]
Jackie Chaney had expected that the verdict would be unfavorable to her client. [to avoid the misreading Jackie Chaney had expected the verdict…]
First-quarter results suggested that a weak economy was having little effect on the company’s performance. [to avoid the misreading First-quarter results suggested a weak economy]
This caveat about clarity should not be taken as grammar and style law. Out there in the world of business writing are plenty of good that-less sentences, structurally like the three examples above, that are sufficiently clear and that simply sound better without the that. Use your good judgment.