This is what makes Russian grammar terribly difficult. You learn that feminine words end in "а" and you think you got feminine nouns down. And then all of a sudden you come across the word "врага". You can only find "враг" in your dictionary. Then you find out that "врага" is not a female "враг", rather it is the genitive of the same "враг"!
Please be assured that it's going to take you quite a while to get used to this thingy. You not only have to learn new words, but also have to learn their gender and all their six cases. Thus, Russian is six times harder than English to learn.
Took me a year or so. Happy language learning!