Learn Polish: Kupujemy zimną wodę i dużą kanapkę. - We are buying cold water and a large sandwich.

They are the direct objects of the sentence, meaning they are the things being bought. In Polish, when a feminine noun that normally ends in -a is the direct object, its ending changes to -ę. This grammatical change is called the accusative case.

Why do the adjectives zimną and dużą end in -ą instead of -a?

Adjectives must match the grammatical state of the noun they describe. Because the feminine nouns wodę and kanapkę are in the accusative case, their adjectives must also take the accusative form. For feminine adjectives, the normal -a ending changes to -ą.

There is no Polish word for "we" in this sentence. How do we know who is buying?

The verb itself tells you who is doing the action. The -my ending in kupujemy strictly means "we". In Polish, you usually drop the word my (we) because the verb's ending already makes it obvious.

I see the special letters ę and ą at the ends of these words. How do I pronounce them?