"That" is one of the words in the English language that sometimes needs to be in a sentence in order to be grammatically correct and to help the reader understand meaning better, and other times is optional (it is correct to include it or not include it, based on your own writing style).

Here is a sentence that can function with or without "that" for you to consider:

With: The mashed potatoes that I ate for supper had too much butter.
Without: The mashed potatoes I ate for supper had too much butter.

Both sentences are correct, and it is really a matter of style as to whether or not you should include "that." When I wonder about using "that" in a sentence where it is a matter of style, I read the sentence aloud and ask myself, "Does the sentence flow better and help the reader to understand my point when I leave 'that' in?" Often in my writing, the answer is NO (i.e. if it is grammatically correct, I go ahead and remove the "that").

Speaking of grammatically correct usage, there are times when you absolutely must leave "that" in the sentence. These are usually times when leaving out "that" makes the sentence confusing. Here is an example:

Brent maintains David's lawn is too big.

Without "that" in the sentence, the reader is initially led to believe that Brent maintains, as in mows and waters, David's yard. "Maintain" is a word that can mean different things, and the meaning changes with the inclusion of "that." Look at what happens when we add "that" to the sentence:

Brent maintains that David's lawn is too big.

Adding "that" to the sentence makes it clear that Brent has an opinion about the size of David's lawn, not that Brent is taking care of David's lawn. In order to make the sentence clear to the reader, we really do need to add "that."

Another grammatically correct use of "that" occurs in sentences that need to maintain parallel construction. Here's an example:

Brent realized he had left his iPhone on the kitchen counter, and that he didn't feed the cats.

This sentence actually needs one more "that" because Brent is realizing two things. This sentence would be better if we used "that" twice:

Brent realized that he had left his iPhone on the kitchen counter, and that he didn't feed the cats.

The best way to know whether or not to include "that" in a sentence is to ask yourself, "Is 'that' going to help the reader better understand the meaning of the sentence?" Try it with "that" in, and try it with "that" out.