Learn Russian: Вместо говядины мы купили свинину.

The preposition вместо (instead of) always requires the Genitive case. Since the dictionary form is говядина (a feminine noun ending in -а), it changes to говядины in the Genitive.

Why do говядины and свинину have different endings if they are both feminine nouns?

They are doing different grammatical jobs in this sentence. Говядины is in the Genitive case because it directly follows the preposition вместо. However, свинину is the direct object of the verb купили (what did we buy?), so it takes the Accusative case, changing its ending from -а to -у.

Is there a trick to remembering the words говядина and свинина?

Just like in English (cow/beef, pig/pork), Russian often uses different roots for the animal and the meat we eat. A pig is свинья, which directly gives us the meat свинина. A cow is корова, but its meat is говядина (which comes from an old Slavic word for cattle). Notice how meat types in Russian frequently end in the suffix -ина.

Could we put вместо говядины at the end of the sentence?

Yes, you could absolutely say Мы купили свинину вместо говядины. Russian word order is flexible. Putting вместо говядины at the beginning just places a little more emphasis on the contrast, highlighting exactly what was replaced right away.