Biblical Hebrew Forum

[David P Donnelly <davedonnelly1 AT juno.com>]

http://www.jewishencyclopedia.com/index.jsp
The online Jewish Encyclopedia, in its article “Jehovah”
gives its explanation concerning why a shewa,
and not the composite shewa [hatef patah]
appears in Y:HOWFH:
>>>
The use of the composite "shewa" "hatef segol" in cases where "Elohim" is
to be read 
has led to the opinion that the composite "shewa" "hatef patah" 
ought to have been used to indicate the reading "Adonay." 

It has been argued in reply that the disuse of the "patah"
is in keeping with the Babylonian system, 
in which the composite "shewa" is not usual.

But the reason why the "patah" is dropped 
is plainly the non-guttural character of the "yod"; 

to indicate the reading "Elohim," however, 
the "segol" (and "hireq" under the last syllable )
had to appear in order that a mistake might not be made and "Adonay" be
repeated. 

FWIW
Dave Donnelly