What's going on when words like "what" and "how" are used in the following, non-interrogative and relative-pronoun, way?
"Oh, [how] kind of you! My, [what] a nice young man you've become, love!"
Now, unless I'm an idiot (which is possible,) may I ask what these words are? And what they're doing? As I don't quite understand, and have recently began noticing them popping up more and more in my writing.
I also know these can't be relative-pronouns, as in Japanese, relative-clauses are made by putting the clause behind the noun-phrase being talked about:
"The student went home today." -> "To be the student who went home today."
This meaning of course that Japanese doesn't have relative-pronouns. However, Japanese still has these non-interrogative "what" and "how" words, one example being the adverb 何と (nanto).
So ya, what exactly are these words? What exactly do they do to speech? And why do they all seem to be, or be made from, originally interrogative words (何 means "what?")?
I've been looking and haven't found much, thus I ask.