"Oni piją kawę."

Translation: They are drinking coffee.

[Danny326737]

Is their an easy way to remember when to end ę or ą with in these situations?

[Jellei]

For "kawę"? This is Accusative, for the direct object of the sentence.

Feminine nouns end with -ę. So does the word for "this": "tę".

Feminine adjectives, pronouns (so mostly possessives) and the word for "that" ("tamtą") end with -ą.