Learn Russian: Девушки купили много ягод. - The young women bought a lot of berries.

The word много (a lot) requires the noun following it to be in the Genitive case. Since there are multiple berries, we need the Genitive plural. The dictionary form is ягода (a feminine noun ending in -а). To form the Genitive plural, we drop the -а entirely, giving us the "zero ending" form ягод.

Why does the plural word девушки end in -и rather than -ы?

This is due to the 7-letter spelling rule. The singular noun is девушка. Normally, feminine nouns ending in -а change to -ы in the plural. However, Russian spelling rules dictate that you can never write the letter ы after к (and six other specific consonants). Therefore, it must be written with an -и.

Could I use покупали instead of купили here?

You could, but it would change the meaning. Купили is a perfective verb, meaning the action is a single, completed event with a result (they bought the berries and now have them). If you used the imperfective verb покупали, it would mean they were buying berries (focusing on the process) or that they used to buy them regularly.