[Grammar] what clauses

[hhtt21]

The title may be wrong. Would you please explain how to use constructions in the form of what + clause?

I have no idea what you are trying to say.

Don't you use idea with the preposition about? So should not the sentence be I have no idea about what you are trying to say.

Would you please explain these grammatically?

 

[PaulMatthews]

I have no idea (about) what you are trying to say.

The underlined element is called a subordinate interrogative clause (embedded question) functioning as complement of “idea” (or of "about" if it is included).

The preposition “about” is optional.

Main clause interrogatives are characteristically used to ask questions (c.f. What are you trying to say?). Subordinate interrogatives on the other hand express questions but do not themselves ask them. Usually the construction can be glossed with the formula "the answer to the question", thus:

"I have no idea of the answer to the question ‘What you are trying to say?'”