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Russian, an adverb is required to describe someone’s language abilities, e.g. "Они говорят по-испански", "Я немного понимаю по-французски" while nouns are used to describe nationality, e.g. 'Это японцы' and adjectives are used to describe a corresponding language, as in “Это испанцы. Я немного понимаю испанский.” Make sure that you know the difference between adjectives of nationality and corresponding nouns: in most cases, the forms of nouns and adjectives will b different as in испанец, испанский язык, but in some cases they will completely overlap, as in русский, русский язык. Remember that Russian nouns have grammatical gender. This is why all nouns for nationalities have a feminine and a masculine form, for example: русский – русская, японец – японка, испанец -- испанка