Translation:Nine days are long.
What kind of conjugation is that? Not even polite, formal or casual but instead in the dictionary form
It is a short way to say 이다. When you say ...다 people will automatically understand the 이 before it.
You'll often hear 바다다! in Korean shows. (Oh the sea!)
That makes no sense. 200 bones equal a skeleton, but we don't think of bones as a collection.
Three kilometers is a long distance. Again, we don't think of kilometers as a collection.