Learn Finnish: Ostin perunoita kaupasta. - I bought potatoes from the shop.

It is in the partitive plural because it refers to an unspecified quantity, meaning "some potatoes". If you were talking about a specific, known batch of potatoes (like "I bought the potatoes we needed"), you would use the accusative plural form perunat.

How does the base word peruna change into perunoita?

To form the partitive plural, you need the plural marker -i- and the partitive ending -ta. When a word like peruna ends in -a, that final -a often changes to an -o- before the plural -i-. So, peruna + i + ta becomes perunoita.

Why does kaupasta only have one p when the base word is kauppa?

This is an example of consonant gradation. Adding the -sta ending closes the final syllable (kau-pas-ta). In Finnish, a closed syllable usually causes a strong consonant combination like pp to weaken into a single p.

Could I use the ending -lta instead of -sta to say "from the shop"?