Learn Polish: Kupiłam dużo ziemniaków. - I bought a lot of potatoes.

In the Polish past tense, the verb ending changes based on the gender of the speaker. Kupiłam (ending in -am) tells us the speaker is female. A male speaker would say kupiłem.

Why is the word for potatoes ziemniaków here instead of the standard plural ziemniaki?

In Polish, words expressing a quantity, like dużo (a lot), mało (a little), or trochę (some), force the following noun into the genitive case. Since there are many potatoes, we use the genitive plural form: ziemniaków.

How do we get the ending -ów in ziemniaków?

The base word is ziemniak (a masculine noun ending in a consonant). To form the genitive plural for most masculine nouns, you simply add the ending -ów.

Can I use kupowałam instead of kupiłam in this sentence?

You could, but it would change the meaning. Kupiłam is a perfective verb, meaning the action is completely finished (I bought). Kupowałam is imperfective, which focuses on the ongoing process (I was buying or I used to buy).