A simple technique I use to improve my writing is avoid the use of the English word 'that'. If you find yourself just having written a sentence with this word in it, either remove it or rewrite it. As always, there are exceptions to the rule. However, the vast majority of the time your sentence will turn out better. Try it and see what you think.
An Indicator You Need to Rewrite the Sentence
If I see a sentence with this word's presence, I immediately think, "can I rewrite this sentence in a more effective manner?" Consider the following sentence I recently drafted for a technical article.
A strong motivational factor engrained in any DevOps approach is that engineers will be responsible for the operations of code they write.
When I first re-read the sentence, I was not pleased. The word 'that' serves as the direct object. The basic subject-verb-object sentence structure is "factor-is-that". This pattern is a clear indication to restructure the sentence, as I did below.
The DevOps approach motivates engineers to write higher quality code because they will be responsible for it's maintenance.
Now the basic sentence structure is "approach-motivates-engineers". Why is this better?
It is much clearer. After only the first five words of the sentence, the reader understands the point. The dependent clauses serve to add further detail.
We removed some unnecessary words. The word count went from 22 to 18. From our last article on avoiding run-on sentences, we learned this is now in the desired range for sentence length.
A classic example of where I simply remove the word is the following sentence.
I was told that it takes light from the Sun eight minutes to arrive at Earth.
Now read the sentence with the word removed. In my opinion, the latter is nicer and cleaner. The word 'that' serves no purpose.
I was told it takes light from the Sun eight minutes to arrive at Earth.
So When Would You Use It?
There are a indeed a few cases where it is appropriate. Most proper usage occurs in conversation as opposed to writing. 'That', and its plural form 'those', are used as determiners to clarify which items the speaker is referring to. For example, if you were walking through a museum and entered the Impressionist wing of the building, you might say "That is my favorite work by Monet." There could be multiple paintings by Monet in the room, and thus you need to clarify which specific painting you like.
However, this use case is less common in writing. You are using words to describe the whole scene and figuratively paint the picture, so often its best to explain the items in question. Again, the word 'that' is likely unnecessary.
As a subject of a sentence, it can be useful as a shorthand reference to a previous item. A pronoun subject is a valid use case. Consider the following prose, extended from a previous example.
I was told it takes light from the Sun eight minutes to arrive at Earth. That is truly amazing.
Well, we did find a valid case for the word 'that'. At now least I've shown I haven't completely eliminated the word from my vocabulary.
However, it is way overused and I would offer it's usage is an indicator it's time to reflect on what you just wrote. See if your sentence can be restructured. Try removing the word completely, and if the sentence doesn't make sense, then restructure the subject-verb-object to be cleaner. Your reader will appreciate you for it.