If the singular form ends in ь the plural form ends in и.
You can read more on this here: http://learningrussian.net/introducing_people_grammar.php
According to this Wikipedia page https://ru.wikipedia.org/wiki/%D0%A2%D0%B5%D1%82%D1%80%D0%B0%D0%B4%D1%8C, it's Greek: τετράδιον — one quarter of a sheet, from Greek τέτρα — four. I presume this refers to the fact that each side of a page in a traditional stapled notebook would be a quarter of the surface of the original sheet, but that's just my guess.
i believe the Russian course isn't as well put together... basically speaking it's more odd a thing for words... to learn grammar or speech you need to find videos or you can say a Russian word whilst recording yourself and compare how that sounds to the Russian pronunciation then tweak it until it sounds right... and for grammar go to russianpod101.com it helps a lot... anyways good luck with learning grammar