Biblical Hebrew Forum

[E <ej2500 AT gmail.com>]

B"H -

I have been studying Davidson's Lexicon of Hebrew, in particular the "declension" of nouns they have arranged. 

I am stuck on a concept I can't figure out: in the 2nd declension, which refers mainly to reduction of kamets, they point out that when the grave suffix -chem is added to two words, namely 

YAD and DAM, the resulting forms are YED-CHEM and DIM-CHEM. 

They say that a patach does not result, because the forms segol-shva and hiriq-shva are shorter than the expected patach-shva. 

I am confused as to what this exactly means - what makes one construction shorter than another, when all are short vowels? 

How also, can vowels shorten out of their vowel types, e.g. a kamets moving to the e type segol, and also the i type hiriq? 

I was under the impression vowels do not switch into other vowel types - the reduction is strictly within their own boundaries. 

I am very curious about this question, and subscribed to this list on its merit. I thank anyone who can help. 

Best, 

Aviel Jones