- I don't know [who's responsible].
- I don't know [who's the best person for this job].
- 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:
- I don't know who responsible is.
- I don't know who the best person for this job is.
- 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?