Good Point. Whether the root here is XWH ,
and the Binyan is ISTF(L as attested Ugaritic,
and still existing in Arabic, or $XH, it is simply
a variation of verbal endings in the 3rd pl.
perfect in the case of L"H roots.
Uri Hurwitz
2. the word you chose is one of the most difficult and irregular. i can
provide several contradicting conjectures to its origin. however, it is clear
that the transformation H$TXWH --> WY$TXW is the so-called "short
form" obtained by removing the final H, just as $TH --> WY$T or
BKH --> WYBK.
conclusion: the -U is not a suffix here. it appears to be
a root letter of an irregular root.