[Grammar] What is the difference in usage between "so", "it" and "that”?

[on the way]

What is the difference in usage between "so", "it" and "that”?

First, please look at the sentence---"Guess the meanings of the following words and phrases from word-formation rules or context clues. If you fail to do so, look them up in a dictionary.” My question is whether "so" can be replaced with "that" or "it" in that context? What's the rule or rules in differentiating the usage of "it", "that" and "so" in contexts like the one above. My understanding is that "it" is chiefly used to refer to an object mentioned above, while "that" and "so" can be used to refer to an action stated above. But I'm not sure about it. Any reply will be much appreciated. Thank you all.

 

[probus]

Re: What is the difference in usage between "so", "it" and "that”?

"That" and "it" are intelligible but unnatural with "fail to do."

You could say "If you are unable to do that (or it)" and those would be more natural. There are no rules defining what is natural and what is not. It is simply a question of what fluent speakers usually choose to say.