Plurals with pretonic reduction (part A)
A. Two-syllable nouns with a closed first syllable (ex. מִזְבֵּחַ altar becomes מִזְבְּחוֹת) When the plural ending is added the accent moves back, because a closed, unaccented syllable is always short, but a shewa cannot reduce, the pretonic reduces. (Rule: if a propretonic cannot reduce, an i-class or u-class open pretonic often will reduce).