Learn Russian: Мы купили говядину вместо свинины.

The preposition вместо (instead of) always requires the Genitive case. Since свинина is a feminine noun ending in -а, its Genitive form ends in -ы, making it свинины.

Why is говядину in the Accusative case, but свинины is in the Genitive case?

Cases are determined by a word's specific job in the sentence. говядину is the direct object of the verb купили (the thing being bought), so it takes the Accusative. However, свинины immediately follows the preposition вместо, which strictly dictates the Genitive case regardless of what the main verb is doing.

Is there a pattern to the Russian words for meat, like говядина and свинина?

Yes! The suffix -ина is frequently used to turn an animal into its meat. For example, свинья (pig) becomes свинина (pork), and курица (chicken) becomes курятина. говядина is a slight exception, as it comes from an old Slavic word for cattle rather than the modern word корова (cow).

Could we use the imperfective verb покупали here instead of the perfective купили?