How do you know when to use ‘mikä’ and when to use ‘mitä’ in a question: “____” se on?
The answer has to do with the partitive. If I’m expecting the answer to be in the partitive, then I can use ‘mitä’ because that’s the partitive form of ‘mikä’.
If I have a bottle of drink in my hand and I want to know what the liquid inside it is, I ask: “Mitä tämä on?” and the answer (“water”) is probably going to be in the partitive case: “Vettä” instead of the basic form “vesi”.
However, if I’m holding the bottle and I ask “Mikä tämä on?” then it’s a different situation. Mikä will signal that I want to know what the item is! In other words, I want to know what the countable item in my hand is. Therefore, whoever replies to me, will answer something like “Pullo” (“a bottle”).
Watch this video to get the full explanation!