I am trying to figure out head-internal relative clauses. A paper I looked at presented two versions of the same sentence, one with the head word (りんご) outside the relative clause:
[皿の上にあった]りんごをくすねた。
And one version where it is inside the relative clause:
[りんごが皿の上にあった]のをくすねた。
Do these two sentences mean exactly the same thing, or are there differences in nuance? When are head-internal relative clauses usually used in Japanese?
They are slightly different, if not much. The former sounds saying a fact relatively objectively.
On the other hand, the latter rather means "although an apple was on the plate, s/he stole it" and it sounds somehow accusive in the sense that it should have been there. In grammar for old Japanese, a similar form is considered a conjunction.
When are head-internal relative clauses usually used in Japanese?
I forgot to answer to this part. Speaking of this usage of を, not specifically to "head-internal relative clause", I personally use it like a weaker version of のに in the point of paradoxical sense, mostly in the form of "…、それをさぁ～、…。". So, I think I've been using it to some degree. I personally am used to the form of HIRC with を too. But I'm not sure other native speakers share that sense.
I could have the wrong end of the stick here, but your second example doesn't make sense to me.
Let's look at your first example first.
[皿の上にあった]りんごをくすねた。
At its core (haha, pun not intended), we have:
りんごをくすねた。
I pilfered the apple.
The relative clause portion tells us more about the apple:
[皿の上にあった]
[(it) was on the plate]
Looking at the whole utterance, we get:
[皿の上にあった]りんごをくすねた。
I pilfered the apple [(that) was on the plate].
Now let's look at your second example.
[りんごが皿の上にあった]のをくすねた。
The core here is more complicated, because we don't have a simple concrete noun as the object of our verb くすねた. Instead, we have the の, which here is used to nominalize (make a noun out of) the entire preceding clause.
So let's look at the embedded clause.
[りんごが皿の上にあった]
[The apple was on the plate]
Okay, simple enough.
After this, though, we have that の, turning our entire embedded clause into a nominalized phrase. This can be a bit messy to translate into English; it comes through somewhat similar to "the fact that", or "the act of", or sometimes by turning a verb into the "-ing" form. Some examples:
ラーメンを食べるのが好きです。
I like (the act of) eating ramen.
Note that this is different from just "I like ramen". We're not talking about "ramen" as the main noun, but rather about the whole clause that contains "ramen" -- in this case, about "eating" it.
道子さんが東京に行ったのは知らなかった。
I didn't know (the fact) that Michiko went to Tokyo.
Again, this is different from "I didn't know Michiko". We're not talking about "Michiko" as the main noun, but rather about the whole clause that contains "Michiko" -- in this case, that she "went to Tokyo".
Looking again at the whole second example sentence then:
[りんごが皿の上にあった]のをくすねた。
The key is that it turns the whole embedded clause into a kind of noun: we're not talking about "the apple" anymore, but rather the fact that "the apple was on the plate".
Because of the verb here, くすねた / "pilfered", nothing quite makes sense -- just due to the meaning of the words, this doesn't fit together. The best translation I can come up with would be something like:
I pilfered (the fact that) [the apple was on the plate].
...??? That doesn't make sense in English. Nor does the Japanese make sense. (At least, as I understand it.)
If you change the verb from くすねた to 見た, that would work:
[りんごが皿の上にあった]のを見た。
I saw (the fact) that [the apple was on the plate].
The paper in question appears to be this one:
This seems to have been written by a native speaker of Japanese, which makes that second sample sentence a bit of a head-scratcher for me.
For the verb くすねる, I'm only aware of the sense "pilfer, filch, sneak, pinch, swipe", with the core underlying meaning of "to steal something sneakily". There might be a sense of くすねる that I'm missing, which could make the second sample sentence work better.
