Learn Polish: Świnie jedzą ziemniaki.

Most feminine nouns change their final -a to -y in the plural. However, świnia ends in a soft -ni sound. In Polish, feminine nouns ending in soft consonants take an -e in the plural instead. Therefore, świnia becomes świnie.

What is the infinitive form of jedzą, and why does it look so different?

The infinitive is jeść (to eat), which is a very irregular verb. While it changes a lot in the present tense (like ja jem, ty jesz), the "they" form is jedzą. The -ą ending is the standard present tense ending for the third-person plural ("they").

Why does the plural of ziemniak (potato) end in -i?

Ziemniak is a masculine inanimate noun. The standard plural endings for these are -i or -y. Because ziemniak ends with the letter -k, Polish spelling rules dictate we must add -i (since the letter combination "ky" is generally avoided in native Polish words). This gives us ziemniaki.

Ziemniaki is the direct object here. Doesn't the accusative case usually change the noun's ending?