Melchert in "Hittite h_i-Verbs from Adverbs"
http://www.unc.edu/~melchert/krakow.pdf
concludes that the Hittite h_i-verb stems *a:ppai-, *p(a)ra:- and
*s^anna- are derived directly from eponymous adverbs, ie. the h_i
conjugation endings are suffixed directly to the adverb. By some
strange coincidence those adverbs, cognates of Latin sub, super, sine,
are the only non-verbs (apart from a number of mainly Germanic nouns)
to occur with s-mobile. This seems to indicate that the place in the
sentence where they incurred that s-mobile was that of the verb, and
nowhere else.
Torsten