The difference between tai and vai
Both tai and vai mean "or"
When using tai, you are allowing your listener to pick whatever number of possibilites they would like.
|Haluatko sokeria tai kermaa?||Do you want sugar or cream?|
|The listener can take cream, sugar, both or neither|
|Isä, saanko jäätelöä tai suklaata?||Dad, can I have icecream or chocolate?|
|Dad can say icrecream, chocolate, both or neither|
You are making your listener choose between the possiblities you offer.
|Haluatko kahvia vai teetä?||Do you want coffee or tea?|
|The listener can take coffee, or tea. You are ruling out that he/she takes both of them.|
|Kumpi on vanhempi kaupunki, Tampere vai Turku?||Which is the older city, Tampere or Turku?|
|The listener can either pick Tampere or Turku, but not both.|
Vai also has a second meaning: the beginning of a short (not very enthousiastic) reaction to something said, asking for a confirmation.
|Vai niin.||Oh, like that.|
|Vai teetä.||So, tea.|
|Vai mitä?||Right? Isn't it?|