Learn Polish: Kupuję smaczną zupę. - I am buying a tasty soup.

In Polish, when a feminine noun like zupa is the direct object of a sentence (the thing being acted upon—in this case, the thing being bought), its ending changes from -a to -ę. This is called the accusative case.

Why does the adjective smaczną end in "-ą" instead of "-ę" like the noun?

Adjectives and nouns have different ending rules! For feminine direct objects, nouns change their -a ending to -ę (like zupę), but the adjectives that describe them change their -a ending to -ą (like smaczną). It's very common for learners to mix these two up at first.

Where is the word for "I" in this sentence?

In Polish, you usually leave out the word for "I" (ja) because the verb itself tells you who is doing the action. The -ę ending on the verb Kupuję specifically means "I am buying".

Does Kupuję mean "I buy" or "I am buying"?

It means both! Polish doesn't have a separate "-ing" tense like English does. Depending on the context, Kupuję smaczną zupę can mean "I buy a tasty soup" (as a general habit) or "I am buying a tasty soup" (right now).