Learn Polish: Kupujemy nowe kurtki i czarne spodnie. - We are buying new jackets and black trousers.

They are in the Accusative plural form. Since they describe non-virile nouns (a category that includes all inanimate objects like clothes), the plural adjectives take the -e ending.

If the singular word for jacket is kurtka, why is the plural object kurtki instead of kurtky?

Feminine nouns ending in -a usually take -y in the plural. However, Polish has a strict spelling rule: after the letters k or g, you must use -i instead of -y. That is why kurtka becomes kurtki.

I learned that kurtki and spodnie are the Nominative (subject) plural forms. Why are they the same here when they are the object of the sentence?

That is the best part about this grammar rule! For the non-virile category (anything that isn't a group of men), the Accusative plural (direct object) is exactly the same as the Nominative plural (subject). They don't change form.

Is spodnie always plural, like "trousers" in English?

Exactly. Spodnie is a "plural-only" word in Polish. Because it is an object, it automatically falls into the plural non-virile category, which is why it pairs with the plural adjective czarne.