About what are you talking?

Correct. But I would use #2 only to emphasize my confusion. Otherwise, it's a bit awkward.

As I've known, 1 and 3 are used in daily informal spoken conversations or in situations that people are friendly. We can also use 2 and 4, but these are formal forms, you can use them in formal writings.

Gigilian NguyenWe can also use 2 and 4, but these alre formal forms, you can use them in formal writings.

No, as far as common English goes, even at conversation level, # 2 and 4 are dreadfully incorrect. Don't use it.

Oh, really? But how comes my grammar book said that those were right, but just speacial. Okay, I won't use them, at least in daily conversations. Thank you!

moon72963. What are you turning on? (air conditioner)
4. On what are you turning?

You could make this inversion (although the inverted sentence would be awkward and not commonly used) if the original were something like "What are you standing on?", which would become "On what are you standing?" But you can't separate the parts of "turning on" like that.

khoff "on" is part of the verb (I suppose it's a "phrasal verb"), not a preposition of location.

My students understood very well when I simply called it a two-word verb.

I guess it's at the end. Same with for example:
Who is she dancing with?
With who is she dancing with?

<p>or:</p>
<p>What are you looking for?</p>
<p><span style="text-decoration: line-through;">For what are you looking?</span></p>

in response to louiST post:You cannot use "who" after a preporsition such as "with." With whom is she dancing? is correct, albeit formal. In casual conversation, you would say "Who is she dancing with?"

You started a sentence with the word but... Shhhhh ;P