Sometimes rules don't help.

The other day on Lang-8, I answered a post about when to use "it" vs. "that". I think the distiction this poster was asking about is the type of difference that

• takes a long and complicated explanation
• has a lot of exceptions to the rule
• doesn't cause much confusion when used differently from how a native speaker would use it

And therefore should be learned by memorizing specific examples rather than trying to apply a set of rules.

Here's the question:

I don't know well how to use "that" and "it" correctly. When I was in Canada, I came across the expression "You can't do that!". I'd like to consider the "that"/"it" distinction through that expression.

Situation: One day, my friend and I went to a coffee shop together. I purchased a cup of coffee there but he did not because he had his coffee with him which he had got somewhere. When he tried to use sugar at the coffee shop, a salesperson said to him, "You can't do that!"

Question: In this situation why is "it" not correct ? (why can you not say "You can't do it!" ?)

I asked several English speakers about this question, but the question seemed to them to be difficult to answer, though intuitively they knew "that" was absolutely correct.

And here was my response:

Hi tsurubun,
I found this topic interesting. Here's my opinion, based on teaching, learning and thinking about language for a few years.

The reason people can't explain why you use "that" instead of "it" in the phrase "you can't do that" is because people don't choose which word to use based on reasons. Instead, they hear a lot of people around them saying "You can't do that!" over and over again, and so they learn to say it that way. Then they learn another phrase that uses the word "that", and then another, and then another. This is how people learn to understand what "that" means.

Sometimes it's useful to try to understand the reasons for using one word instead of the other. But other times, it's just better to remember the correct form and accept that that's the way to say it. One day you might learn some other phrase using "that" and think, "Oh, this is just like the phrase "You can't do that." Now I see!" But for now, whatever explanations you read on this site probably won't help you to use "that" and "it" better.

What do you think of this idea?

I know it's probably annoying for me to answer this way when someone is asking questions about grammar. I do try to answer when I think that knowing a rule could be a shortcut. But in cases where there aren't any shortcuts, I can't help but point out that sometimes you just have to learn each case one by one.

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