Sometimes, the smallest words make up the most interesting errors. A common speech error I’ve noticed among ESLers, is the addition/placement of “it” as the object in a complement. For example, consider the following:

*1. Your questions are always hard to answer it.
*2. What you just said is something that we cannot do it.

(Linguists conventionally head an unnatural sentence with a [*], and a questionably/regionally unnatural sentence with a [?])

In both cases, the “it” at the end is superfluous, because either (a) the whole complement (ie, the bit that goes after “is/are”) is used to describe the subject (the bit that goes before the “is/are”), and so it would create a recursive referent (e.g. “your questions are hard to answer themselves”); or (b) because it’s part of the subclause modifying the noun (e.g. “questions {that are hard to answer}).

But admittedly, this is a relatively high-level error. After all, consider the following related sentences:

3. You’ve asked the question to me five times, and I’ve tried to answer it the best I can.
4. There was a jingle in her pocket, and she reached to answer it.
5. I only have one question, and you haven’t been able to answer it.

Moreover, there are sentences that omit the (optional) object:

6. It was a moment before I could bring myself to answer [her question].
7. Nevermind, you don’t have to answer [me].
8. At first, he didn’t think the vampire was going to answer [him].

But of course, there are sentences, in which no object is implied, because it resides as the subject of the sentence:

9. Boy, that’s a tough question to answer.
10. You know, that is a question that the president would have to answer.
11. The words were coming so fast it took me a second or two to realize she had, somewhere in there, formed a question I was expected to answer.

Straight-forward, right? I would love to read up on any research that’s been done in this area…

[note: all sentences drawn from COCA]