The following sentence was on one of the tests:
What would you like to do that others have told you is impossible.
Students have asked why that could not be replaced with what. I.e.,
What would you like to do what others have told you is impossible.
We have been trying to debate, we know it doesn't sound right but they expect a more detailed explanation, will really appreciate if anyone has an answer for this.
"That" can introduce an explanatory dependent clause. "What" cannot. "That" indicates a specific quality of the object in question, or a subset of a larger set. "What" indicates the object in question itself, or the entire larger set. (e.g. "What was the bird you used to own that was red?")
In your example, "what" is incorrect because it creates a logic problem; it establishes an equivalency between the two clauses. The second clause, however, is not equivalent to the first; it explains the first clause. "What would you like to do?" is a reference to the entire set of all possible things you would like to do. "That others have told you is impossible" specifies the particular characteristic of the subset the questioner wants to identify.
Notice how I can restructure the sentence so that the two clauses become substantively and logically equal, and THEN the word "what" would be correct, thus: "What you would like to do IS what others have told you is impossible." (I emphasize "is" to point out the equivalency.)
A rule of thumb here, not perfect grammar I know but rather a working explanation for intermediate level ESL.
"What..." often works like a single noun:
I know **the answer**.
I know **what you need**.
He always does **the work**.
He always does **what she wants**.
You could replace the "what" clause with "something": "I know something", "He always does something".
In contrast, "That" joins two sentences together, replacing part of one sentence.
This is the house. Jack built it. This is the house that Jack built.
This is the cat. The cat killed the rat. This is the cat that killed the rat.
You can't replace the "that" clause with "something". You can't say, unless drunk or gibbering, "This is the cat something."
You can say: "Would you like to do something?", and so you could also say "Would you like to do what others have told you is impossible?"
But you can't say "What would you like to do something?" You'd just say "What would you like to do?" So in this case, a "what" clause doesn't work.
