I can understand most of the breakdown of the word מוסרותימו in Psalm 2, thanks to your website viz masculine noun from root אסר with female plural ending ות. Also the poetic ending מו for 'their', but could someone please explain the tsere yod?
I've just been researching pronouns, which I've always had problems understanding, and came across the answer to my own question! The tsere yod being the link between a plural noun and the suffix (in this case 'their'). Hoping perhaps that finding the answer to my confusion will help someone else.