The problem with どれか？ which the other comments seem to have missed is that it has its own different meaning.
It actually follows a pattern all question words (to my knowledge) use in Japanese. These examples are a bit easier to understand intuitively:
・何 = "what thing", 何か = "something"
・どこ = "where", どこか = "somewhere"
・いつ = "when", いつか = "some time"
In the same vein, どれか means "one (of them)" and is not a question word "which". When you say "dore ka?", it sounds like you're confirming you heard them correctly telling you to choose one.
Yes, when you attach it to question words strangely enough. To make my previous point a bit clearer, let me illustrate the difference between どこ and どこか:
「どこに行きますか？」= where are you going? (Open-ended question)
「どこかに行きますか？」= are you going somewhere? (Yes/no question)
There are many other situations where adding か doesn't make it a question (for example, it can be used as a particle to mean "or" in lists), but in most cases, putting か at the end of the sentence will make it a question.
You might be able to use どれか, as long as you recognize that the か basically takes the place of the question mark in the English translation. I don't hear this often, though.
If you're more concerned with speaking than with formalities, then simply saying どれ？ would suffice. Or my normal personal choice, even if it's technically not right, is どっち？This is also used fairly often. Both are pretty informal, the latter more so.
A couple others have commne ted on it, but a large part of the "desu" is a formality thing. Like someone suggested, just saying dore? Shoukd be fine, but dont just cut out the desu. It can seem very rude/impolite.
Politness is a huge part of japanese culture, so being careful when choosing polite form or casual form is v imortant. A lot of the time the way you speak identifies "who" you are. Like if youre a yakuza member...
Absolutely, but a sentence with a verb is quite different from an exclamation of some sort without. If there is one thing Japanese does much more thoroughly than English, it is mark levels of politeness, and here there is a polite form of the verb to be. I really think, therefore, that a complete, at least neutrally polite, English sentence, rather than some sort of brusque outburst, should be an accepted translation.
I think it has to do with the nature of です as a copula, like "is"/"am" in English, and as a kind of intransitive verb.
As a copula, its function is to simply connect the object back to the subject. In our case, the subject is implied, by the absence of は, to be "the one that you want". です connects the object, in this case どれ or "which", to that implied subject, i.e. "as for the one that you want, which is it?" (what です adds)
As for why there isn't a particle between どれ and です, since です behaves like an intransitive verb, it doesn't work with a direct object particle, and you can't put in は or が because they both specify the subject thus removing the object for です to work on.
どれにする means "which one will you have/do" and indicates the listener having an independent choice of which to do. Also, する is a plain form verb, and thus less formal/polite than です.
On the other hand, どれですか means "which one (is it)" and indicates that the listener can only identify the equivalence between the implied topic and the actual thing.
That probably doesn't make much sense though, so consider this example:
You walk up to the counter of a fast food restaurant, and the cashier tells you they only have two items on their menu, Lunch Set A and Lunch Set B.
"Which one do you want?" = どれにしますか？ (lit. "Which one will you do", note: します is the polite form of する)
Unhelpfully, they don't have pictures on their menu, and your Japanese reading ability is limited, so you want to clarify before you order:
"Which one has a hamburger?" = ハンバーガーのはどれですか？ (lit. "Hamburger's (one) is which one")
They're both acceptable, but they're used in different contexts.
どれですか is more like "which one (is the correct one)?" or "which one (is the one you were referring to)?"
On the other hand, どれにする is more like "which one (will you decide on)?" This is a relatively casual way of speaking; the polite/formal version is どれにしますか. The verb, する and します, means " to do", so you can also think of it as "which one (will you do)?"
For the Kanji, you will need outside sources. There are thousands of them to be used daily, and reading/writting without them would be much very confusing.
As for the Kana, both of the sillabaries used, Duo covers them in the first lectures. With a little practice you will understand them well.
Lastly, you already read Romanji! xD
DoCCHI desu ka? http://yesjapan.com/YJ6/question/2571/what-is-the-difference-between-dore-and-docchi . I recognize "which one" doesn't necessarily refer to two. But without anything in mind, when I hear "Which one?" I think of someone holding something in either hand.
ます isn't a word by itself. It is like adding "ing" to the back of English words to make a word present or future tense. So, the generic verb "to run" becomes "running." です (desu), not てす (tesu) is used when there is no explicit verb in the sentence- recall Japanese use sentences English speakers would regard as incomplete. か at the end of a sentence basically serves to emphasize the sentence is a question.
Both です and ます are honorifics made to make a word polite. You can actually just drop です from most sentences to make them informal. There are transformations of verbs that make them present/future tense without using ます. Those become informal. The Japanese employ different kinds of sentences depending on how polite they want to be. Japan is a crowded country, with a history of extreme violence (samurai could essentially just chop your head off if you looked at them wrong, and they used to fight duels to the death just to see who was the better swordsman), where in much of their history they had an emperor who was toothless, and hence fights would break out among neighbouring warlords. They needed ways in their language to emphasize when they were "playing nice." Just like the English have a "Queen's English" and "street English," the Japanese have different ways of speaking associated with who you are speaking to. The initial sentence structure that is taught in most places is for a moderate politeness level. HTH
It's subject object verb instead of subject verb object. For example instead of "I eat sushi" its "I sushi eat" and often the subject is dropped entirely. So, the words are "sushi eat" and it's understood YOU are doing it. If you want to change the subject to, say a friend of yours, then you say "friend sushi eat" and continue from there. Like "clothes buys" is now attributed to the friend rather than you until you introduce a subject again.
Unfortunately, while this is lesson one of formal Japanese classes even before learning phonetic sounds, duolingo skips over this crucial cornerstone of the Japanese language.