Why is hyvää in partitivi here?
Pirtelö on hyvää = milkshake is good. You've had some milkshake and it's good or milkshake in general is good. It's not countable (how much).
Pirtelö on hyvä = the milkshake is good. A countable portion of milkshake.
Out of curiosity, in spoken Finnish, can one really hear the rhythmic vowels??
The milkshake!!! Specific milkshake. Why 'hyvää'?