"Do you have the ticket?"
Translation:Onko lippu sinulla?
I am a native Finn and I do not completely agree. I fully understand the reason for your explanation. However, let's say, you're about to catch a plane and you want to check that your travel partner really has his/her ticket - the specific ticket for that specific journey - it would be natural to ask 'onko sinulla lippu?'.
The original idea of Duolingo was to learn a new language a bit like a child -- by exposure and repetition. And yes, guessing, making errors, and guessing again. This is not meant to be like school, where someone teaches you the theory and the quizzes you on your ability to absorb and apply it. There are plenty of other places for that.
Later, volunteers have added Tips to help get users started. But it's still not meant to be a full-blown language course, spoonfeeding users everything. Asking in here in the discussions is one way of figuring out stuff that isn't clear, searching the internet is another. Uusikielemme.fi is a really good website. And so on.
That might work for others, but not so much for me. As a young child, I was terribly confused about what anyone was trying to say in my own native language, until I went to school and started formally learning the grammar. Maybe it is related to being an Aspie - I couldn't really say for certain. But I find that I need that extra instruction. I'm the worst guesser ever!
this is how Finns can make a distinction between the definite and definite article
Indeed, in this context it is.
In other contexts, Finnish speakers don't make a difference between these two at all. (Which really shows up when they have to write or translate into English or Swedish, where you almost always have to decide which one you mean.)
For example, Lintu lentää can mean
A bird flies.
A bird is flying.
The bird flies.
The bird is flying.