“Do it” versus “do that”

Question

Are there any sentences where do that is preferable over do it?

The New Oxford American Dictionary reports when a phrase is informal; it doesn't do that in this case.

The New Oxford American Dictionary reports when a phrase is informal; it doesn't do it in this case.

Answer 1

This is a subtle point of usage that does not have a clear answer, and I doubt that there are any usage guides that specifically address this issue. However, here's the best I can offer with my native-speaker intuition:

• do it is the most general variation. When in doubt, use do it.
• do that is most used when there's a very specific action in question
• do that can also be used to contrast two different actions, e.g. "Do that and not this."
• As a general imperative, prefer do it.

With the two examples you give above, I strongly prefer the do this variation, as an instance of my second point (we're talking about a specific action).

Answer 2

Well, in these particular cases I think both sound a bit awkward. What you have here in the second clause is (I think) what linguists call pseudogapping, which is a type of verb phrase ellipsis (i.e., the verb in the antecedent clause is left out of the second clause). Sometimes when you leave the verb out of a clause you use "so" as a kind of stand in for the complement or adjunct of the verb (i.e., what comes to the right of the verb in the first clause). In this case, I think "that" and "it" are stand-ins for "when a phrase is informal", but as I said normally in English you would prefer to use "so" for that.

Note that these examples aren't ungrammatical. But they do sound, to my ear, typical of second language English. Not that there's anything wrong with that.

As for the distinction in general between "do it" and "do that", it's subtle. "It" is a pronoun, and as such when you use it you are signaling that you are referring back to something which has already been talked about or somehow referred to in the discourse. (In other words, I can point to something and say "Do it!" and that would be enough.) "That" is a word which you use to point out a particular thing in the world (the technical term is "deixis"), as such "that" is a better choice when you are distinguishing or contrasting one thing from something else.