I don't know who's responsible

Question

1. I don't know [who's responsible].
2. I don't know [who's the best person for this job].
3. I don't know [what's best].

I'm assuming sentences 1–3 are grammatical. How are the parts in brackets analyzed? I know in relative clauses subject–auxiliary inversion doesn't occur, unlike here, so these aren't relative clauses, are they? Are these "fused relatives" or "content clauses" or what? What would a modern analysis be? (I'm not asking for anything detailed here, just the general structure and the relevant terminology, essentially.)

Related to the above:

4. I don't know who responsible is.
5. I don't know who the best person for this job is.
6. I don't know what best is.

Sentence 4 sounds weird/wrong, 5 sounds okay(?), and 6 sounds okay if you're asking about the word "best", I suppose. How come it's sometimes possible to transform the original sentences like this?

Answer 1

In all cases, the elements in brackets are noun clauses, acting as the object of the verb "know". A simpler example would be, "I don't know [something]. The clauses in all three of your examples could be the "something", so they are noun clauses.

Example 2 is not grammatical because you've done sub-aux inversion. "I don't know who the best person for this job is" is correct, and it happens to be your 5th example.

Example 4 is nonsense.

You're right that example 6 could mean you don't know what the word "best" means, and could also mean, "I am incapable of assessing what 'best' would be in this situation", perhaps because you're unqualified, or because 'best' hasn't been adequately defined.

Answer 2

A relative clause is a finite clause that modifies a noun and is a constituent of the noun phrase which has the noun as its head (for example, the umbrella that I borrowed).

Relative clauses can begin with a relative pronoun (such as which, that or who), a preposition and a relative pronoun (for example, to whom, for which) or no subordinator (for example, the umbrella I borrowed).

To identify a relative clause without a subordinator, we can insert a relative pronoun to see if the sentence still makes sense, for example, the umbrella (that) I borrowed.

Ref Grammarpedia

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3. I don't know [what's best].

This is ambiguous because of how "best" can be used. I suggest this should be either

I don't know [what's the best]

the best one or sort

I don't know [what's for the best]

the best decision.

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I would suggest that point 4 through 6 all sound incorrect. Therefore the next three points I will correct

I don't know who the responsible person is.

Only animals or Humans can be responsible, we must use who or that. Also we are being specific so we use the definite article.

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I don't know who the best person would be for this job

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I don't know what would be the best

I don't know what would be for the best

using "would" introduces a possibility "what's the best alternative"

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is; he/she/it form of be Ref C.E.D.