"You have one small dog."
Translation:Sinulla on yksi pieni koira.
Did you report it? That's the best way to see what translations are accepted (and to improve the course as lots of people do it).
My native vote would be no: Sinulla on pieni koira = You have a small dog. The "one" in the English sentence does carry some meaning, giving the sentence a different meaning.
I think it's a little bit confusing here, since in English the "one" can also work as kind of an article to emphasis the adjective. Like "Wow, that is a really small dog!" Whereas in Finnish the "yksi" shows, that you have exactly one small dog. As in "You have one small dog and two big dogs." That's probably why the answer without "one" isn't accepted. Please correct me if I'm wrong.
"On" still means "is" here. But the structure of expressing possession is different in Finnish from what it is like in English. (It's similar to the structure in Russian.)
If you were to translate "Sinulla on" literally, it would be "By/On you is". So the possessed object does not take an object form, because it just "is". There isn't really a straightforward translation of the verb "to have" in Finnish, so you can't translate this sentence word for word, but instead you will learn the Finnish structure.