What's the difference between
Grown-ups can't make drama. If they do "this", they will be like a child
And
Grown-ups can't make drama. If they do "that", they'll be like a child
How can I use these pronouns to replace the abstract nouns?
There is virtually no difference.
Sometimes you might use "this" or "that" in a psychological way: "This behaviour that I am talking about, "that behaviour that you do" and so on. Sometimes "this" and "that" are used just for the first and second idea mentioned.
In the particular example given, "this" or "that" could be used. However it would also be correct not to use a pronoun at all
Grown-ups can't make drama. If they do, they will be like a child.
