Hi, Sarah!
> I didn't ask this question, but I'd not properly understood ablauts until now.
> Your explanation was great. Many thanks.
Actually Berglaug's excellent explanation pertained only to umlauts.
Ablauts are something much more archaic and mysterious :)
Basically the Indo-European proto-tongue is supposed to have had
an alternation between /e/, /o/ and nothing. This alternation left
all sorts of interesting remnants in the descendant languages.
Notably the sound-changes in the strong Germanic verbs are a result
of ablaut.
sing-sang-sung: ablaut
foot-feet: umlaut
Kveðja,
Haukur