How to identify the stem of a Russian word?

Question

I'm using audio CD's in the form of MP3's to learn Russian, and the reading matieral that accompanies it doesn't correspond with the audio, so I end up hounding words out in online dictionaries, which I don't mind, seeing as I've found a guide to tell me what each stem means. I'm just having trouble finding something that'll tell me the stem of the word.

One I'm on now is understand:

Пониматъ, I think that in itself is the stem, but I'm not sure if that is the whole stem, or if it's Понимат- , dropping the sign.

Answer 1

I would turn to Attic Greek.  Most of the roots will be there.  Or, look up etymology, Russian.

Answer 2

At Russian lessons in Russian schools, teachers say that the stem is a part of the word that doesn't change when the word has different cases, tenses, etc. The stem is also shared by words of different class but related meanings. Though it may be not very helpful for non-native speakers. Sorry...

So in понимать  it will probably be поним- (понимаю, понимаешь, понимал, понимание (understanding, noun), понимающий (understanding, adjective)). But the tricky part of it is that понять is also a form of понимать but it looks like the stem has lost some letters (this case is for linguistics specialists, I'm afraid).