"There is a cute bird behind you."
Why is it 可爱的鸟 and not 可爱鸟? I think I still don't understand the full function of 的.
In addition to Jerry's response it's also helpful to keep in mind that 的 wouldn't be required if the adjective were only one character. For example if the sentence were “There is a small bird behind you” then you could simply say 你的后面有一只小鸟. 的 is required here because 可爱 is two characters.
One reason 的 is hard is because there is no functional equivalent in English. In some cases it operates like "of", but in this case it simply indicates a property that belongs to a person or thing. In English you can simply say "a pretty girl" but in Chinese you would say 很漂亮的女孩; it associates the 漂亮 as being a property the 女孩 possesses.
They want an extra 的 between ke ai and niao but in the word bank there is none
As Ray says, "你后面" is acceptable. However in my word bank there were 2 "的s"; they may have changed it so it matches with the "official" translation.
Whoa Ho! I got it right on the first try! Are we learning Chinese or what!? 8/14/18
Yes. In fact, 在你的后面 would be more commonly used than the Duo version, in my experience.