Depends of the situation and the meaning needed) Russian cases correspon rughly (roughly!!!) to English prepositions. 1 nominative Русский язык некоторым людям кажется сложным. (subject. "Russian seems to be...") 2. genitive слова русского языка. ('of'. words of Russian) Я не знаю русского языка (direct object of a transitive verb with negation) У русского языка... (here it means = 'Russian has...'. But often у means 'close to' or 'at'. Anyway у needs Genitive.) 3 dative Я получил тройку по русскому языку! (after prepositions к and по) Я уделяю недостаточно времени русскому языку ('to' Russian language. After some verbs. Gorresponds English to in 'give to somebody', 'it seems to'...) 4 accusative Я знаю русский язык (direct object of a transitive verb. For some nouns accusative coincides with nominatiove. ) Это слово проникло в русский язык из вьетнамского. (after в... and на in the meaning of 'direction'. Correspons to English to, into, onto ) 5. instrumental ...с русским языком... (after some prepositions: c, над, под, за...) Я пользуюсь русским языком при каждой возможности (after some verbs) Я объяснил ему это русским языком. (this use of 'язык' is somewhat rude! corresponds to English 'by' and 'with' in the sense of 'by the means', 'with **** as instrument.) 6. prepositional. о русском языке (after 'о' ='about', and в, на in the meaning of 'location'= English in, on, at) ---- The list is incomlete!