It's a difficult question... В последние пару лет: In this case: пару = несколько = few - В последние несколько лет последние пару дней = последние несколько дней Я купил последнюю пару книг In this case: пару = две = couple, two So, depends on the meaning of this word

accusative case for both words (plural form)

"Пара" in that case is not a noun itself, it's so called numeral (числительное) and you can't divide "пару лет" (these two words together indicate plural form)