You've already met it: Puhun suomea, kaksi euroa, Kahvi on hyvää. That -a / -ä ending is the partitive, and Finns use it constantly. It marks something that is an amount, not a whole thing:

• Food and drink in general: Juon kahvia. — I drink coffee / I'm drinking (some) coffee.
• After numbers above one: kaksi kahvia — two coffees, kolme pullaa — three buns.
• Languages: Puhun suomea.

How to make it:

Base word	Partitive	Rule of thumb
kahvi (coffee)	kahvia	ends in one vowel → -a / -ä
maito (milk)	maitoa	back vowels → -a
leipä (bread)	leipää	front vowels → -ä
tee (tea)	teetä	ends in a long vowel → -ta / -tä
olut (beer)	olutta	ends in a consonant → -ta / -tä
vesi (water)	vettä	irregular — learn it as it is

Whenever English could say some, Finnish reaches for the partitive.