Questions & Answers about Muszę kupić dwie pary dżinsów.
Why do we use dwie pary instead of a collective numeral like dwoje here?
While collective numerals are used for many plural-only nouns (like dwoje drzwi for 'two doors'), items that consist of two identical legs or lenses almost always use the measure word para (pair). Just like in English, you say dwie pary dżinsów (two pairs of jeans) or dwie pary okularów (two pairs of glasses).
Why does the word for jeans end in -ów in this sentence?
It is in the genitive plural case. The noun para (pair) requires the following noun to be in the genitive to mean 'a pair of' something. Since dżinsy is a plural-only noun, you use its genitive plural form, which adds the -ów ending to the stem.
If dżinsy is a masculine inanimate plural noun, why do we use the feminine number dwie?
The number is grammatically counting the word para (pair), not the jeans directly. Because para is a feminine noun, it requires the feminine form of the number two, which is dwie. It functions as the direct object of kupić, so dwie pary is in the accusative case (which looks identical to the nominative here).
Is it ever correct to just say dwa dżinsy?