Russian language is really hard for Westerners to grasp. Russian has, in addition to a very difficult alphabet and complex grammar system to learn, a constantly varying word order, in addition to many sounds that they don't have in English. And plus these „swimming“ stresses in pronunciation of one word by different declension. In English, for example for good knowing of vocabulary one word has some meanings; in Russian, here is otherwise, one meaning has some words.

imperfective and perfective verbs. I never know which one to use :)

грамматика,стилистика,длинные слова=)всё!! но мне повезло,что китайский язык еще труднее)

Thanks. As tenses for russian people. : )

Perhaps the Russian Cases, that are not common in languages based on Latin or Saxon.