Ki is an interrogative - a question word. Using it when you're asking "Who... ?"
Aki is a relative pronoun. Use it for "who" in structures like, "The man who never returned" or "The child who is studying."
Mi and Ami have exactly the same relationship to each other. They're just the inanimate version of ki and aki, really. Mi is "What... ?" and ami is the relative pronoun... although its translation into English is less consistent. It may be translated as "that" or "which" or sometimes "what."
Use ami in structures like "That is what I need" or "Where is the soup that I bought?"
Finally, milyen is an interrogative meaning "What kind of.... ?"
The DuoLingo example is written without punctuation and capitalization. That makes it perfectly clear to me that it is a sentence fragment. And from the Tips and Notes introducing this section, as well as from the examples themselves, it is clear to me that we are drilling relative clauses.
I am surprised at all the griping on this page. Perhaps in an age of cellphone texting, the role of punctuation and capitalization is no longer recognized.
This is a very confusing way to present it. I perfectly understand what it means, how to use it, and that it's part of an incomplete sentence but I have to rack my brain to provide a translation that will be deemed acceptable by the software. Which is plain boring, I'm not here to learn English. Strongly advise to revise this exercice.
"ki" is an interrogative word, it's used for questions: "Ki ül a fa alatt?"/"Who sits under the tree?" "aki" is a relative pronoun, it's used in statements to refer back to a person previously mentioned: "Az az ember aki a fa alatt ül."/"The person who sits under the tree."
The same pattern goes for a whole bunch of interrogative/relative pronoun pairs: "mi?"/"ami" ("what?"/"that which"), "hol?"/"ahol" ("where?"/"in the place where"), "milyen?"/"amilyen" (roughly, "what kind?"/"the kind that"), "mikor?"/"amikor" ("when?"/"at the time when"); "mivel?"/"amivel" ("with what?"/"with the thing that") etc.
If the course designers expect us to be so precise in our knowledge of English punctuation, why, whenever part of a sentence is quoted, are they not using an ellipsis as an indicator? Cryptic hints break concentration thus accurate punctuation would definitely be helpful to all students.
Many here complaining, I'll give it a try in a full sentence. "Aki a fa alatt ül pihen." "Who sits under the tree rests." Is this even possible though? IMHO it is in English, although some do object. This doesn't seem like any of the actual examples which appear on Google. Are they not subject but object clauses? All I can find are rather dependent clauses acting like adjectives modifying the preceding nouns which are followed by commas. If that's the only way this clause can be applied, then that's a lot of missing explanation. At the very least a comma or two as here: ", aki a fa alatt ül(,)" ", who sits under the tree(,)" are begged for. Or maybe Duolingo has the right idea giving these out as assignments and we should just go and find out for ourselves.
for what it's worth, i had to take a second look at it because it was lacking capitalization and question form and it dawned on me that it must be the 'who' that isn't a question. so for me it did its job of teaching me a word by first confusing me but providing enough clues for me to make a guess. Just my view, but in case anyone cares!