Are "joka" and "kuka interchangeable, or do they have a strict use to them?
thats a good question. I guess "kuka" more like "who" and "joka" - "that". And in here joka fits better. But if there are natives here, pls tell, can you use kuka here as well?
Kuka is used in questions, whereas joka in statements (the 'who' being already known).
Thank you for explaining this.
You can't use "kuka" here.
Does the comma have to be there in Finnish?
Yes. It must to be there.
Yes, there is always a comma between the main clause and a subclause in Finnish. Luckily Duolingo does not care about punctuation, since the comma rules are out of scope for a basic course.
So it's similar to: "I have a friend, they know how to draw?"
This now feels like advanced finnish XD