Only the second one is acceptable. IMO
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If you refer to a verb (want), you must use what.Can you expand on this? Where did you get this? What does it mean?
CalifJimIf you refer to a verb (want), you must use what.Can you expand on this? Where did you get this? What does it mean?
I've never heard such a thing.
__________
Only the second version in the original post is correct.
CJ
InchoateknowledgeThere is one thing what they want.
there is one thing that they want
What I must do is write a letter. "What" (and not that) here refers to the verb: write.OK. Got it. I've never heard it stated that way.
This is the letter that I must write. "That" (and not what) here refers to a demonstrative (pro)noun: this.
CalifJim
In what I must do, you might think of it as that thing which I must do or that which I must do.
Maybe it will help. (Maybe it won't! Think of it that way if it helps.)
The what in this structure i