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)
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The list is incomlete!