(i'm term-chanalged in english, so bare with me).
in modern armenian, the plural of one-syllable words is created with adding an եր [er] ending to the noun. for example, տուն - տներ, բառ - բառեր. of course, as a rule, there are some exceptions, such as մուկ [muk], ձուկ [dzuk], մատ [mat] which get ներ [ner] ending, and others such as մարդ [mart] which becomes մարդիկ [martiq].
the plural of multi-syllable words is created with adding ներ [ner] ending.
when the word is a composit word (hope that's the right term), such as գլխացավ [glxatsav], composed of two roots, գլուխ [glux] and ցավ [tsav], the plural of the word is created by adding an ending that you would add to the last root if was on its own. in this example, the last root, ցավ [tsav] is a one-syllable word, which means if it was on its own, the plural would have been ցավեր [tsaver]. thus, the composit word which has ցավ [tsav] as a last root also gets եր [er] as a plural ending.
therefore, գլխացավ [glxatsav] - գլխացավեր [glxatsaver].