Just a minor correction, mashita is the past form.
As far as roots in English and stems in Japanese, I don't really know what you mean either, but here's my guess:
A root word in English is an actual word. E.g. "friend" is a root word for "friendly" and "friendliness." "Regular" is a root word for "irregular." But in Japanese a stem in not necessarily a word. E.g. hashir would be the stem of hashiru. Note that this may not be entirely correct, because in Japanese grammar this sort of stem doesn't exist. Even so, if you say that the stem is hashi, then it still isn't a word (excluding the hashi that mean "bridge," "edge," and "chopsticks," because I'm dealing with the verb hashiru). Does that help in any way?
[Edit]: I just realized where you got that information. Go with the first example that I have given, i.e. hashir as the stem. Note that this is for Group I verbs. Group II verb stems end in either i or e. E.g. the stem of taberu is tabe, also not a word.
