'This' is often used to refer something we just said.
"Sam has won the youth swimming competition for his high school. This made his parents very proud."
Even though it's not wrong to use 'this' in this context, we can write it better by simply replacing 'this' with what it's actually referring to.
"Sam has won the youth swimming competition for his high school. Bringing home the gold medal made his parents very proud."
When we use 'this' in a sentence, it can disrupt the reading flow if the reader has to refer back what 'this' was referring to.
Replacing 'this' with something specific makes the reading experience better because you have spelled it out for the reader so they don't have to make the effort to refer back what you just said.
The same applies to 'that', 'these', and 'those'.
"The school has strict guideline for behaving in the library. Students who do not follow those will be punished.
"The school has strict guideline for behaving in the library. Students who do not follow those rules will be punished."
It's better to be specific, to hold the reader's hands so they don't lose the plot. The more we can do this, the easier the reading experience will be.
There will be cases when we will need to use 'this' and 'that'.
'This' is better used when you're referring to something that's coming.
"This is what I mean..."
While 'that' can be used to refer back.
These are not strict rules however, you should experiment using 'this' and 'that' in sentences and determine whether the reading experience is better with one or the other.
'That' is an overused word, avoid if possible
"He said that he would..."
"He said he would...."
Whenever you use the word 'that', try removing it and see if the sentence means the same. Often it does.
Summary:
- Be specific about what 'this' and 'that' are
- 'This' and 'these are better for something coming; 'that' and 'those' are better for referring back
- consider deleting 'that' where possible