The prescriptive use of です after ない
I'm having sentences marked wrong for not putting です after ない in negative sentences. I was taught years ago that using です in this way is a little uncouth, but even if it is admissible, です is absolutely not required.
I had a question which was something like "It is not big." with several tiles on the bottom to arrange. I picked 大きく and ない and left the です tile untouched because 「大きくない。」, as casual speech, is "more correct" than 大きくないです (the more-polite form would be 大きくありません).
(If you put ん or の after the ない, then adding です is of course acceptable to anyone.)
Now that I think about it, has the word ありません appeared in any of the early lessons?
I know what you mean. I'm actually doing a re-run through the whole tree (on web) and trying to test whether it accepts plain forms of verbs and adjectives. I've almost finished going through all the lessons in the tree in about a week and can answer some of those questions. ^^
Many answers now do actually allow just ない without having to tack です on the end. I expect eventually they all will once they've all been reported as alternative correct answers and fixed. I think it's just that the default answer they gave to most were ない + です and they hadn't added many alternatives to begin with.
ありません is used in the 6th skill in the tree ("Intro 2"), but I don't know if it's used earlier than that point.
A good example sentence from the "Intro 2" lesson 5: "No, I am not fine."
The default answer: いいえ、げんきではありません。
Other answers I typed which were accepted:
I guess it might have been better to have found a 形容詞 (い-adjective) for this example. I got carried away before I thought of this though... Hope this is interesting enough. ^^
I was hesitant to report them because I'm somewhat new to Duolingo (I've only been at it for half a year), but I did report that one.
ないです is certainly used casually by lots of people, but plenty of others think it is ungrammatical and not suitable for being taught as the primary correct form. ないだ is wrong for any native speaker even if ないです is not as wrong. I like how Duolingo allows a lot of variation, but it feels weird to type a 100%-acceptable answer and have it rejected in favor of a form that many natives would reject.
And I would be disappointed if they were teaching this form just because it is easier for non-natives to learn. I wouldn't want to see an English textbook that infantilized learners by prescriptively making them use unnatural English.
Going a step further, what if you put all of your examples in the past tense? The unacceptability of forms based on ないです will become clearer:
With い形容詞 in the past tense,
・赤くなかった (Perfectly fine)
・赤くなかったです (Acceptable? Some natives might reject it)
It looks like です is starting to become acceptable after words that, strictly speaking, should not allow it, but other copulae related to です seem to be more clearly unacceptable.
Here is some discussion on that in Japanese, if you're interested:
My Japanese teacher, a native to Japan, explained ないです as slightly more polite/formal than ない . I'm not surprised that the course isn't consistent at this point, seeing as how it is in limited beta. This is the stage where the course has been released so testing it and finding such inconsistencies can be crowd sourced. :)
To testmoogle, you're awesome! :)
My own Japanese 101 teacher, years ago, actually said that ないです was ungrammatical. For the copula, です contrasted with じゃありません; then in the casual style だ（or zero） contrasted with じゃない。I'm surprised that a native would call a form as disputed as ないです more polite.
She considered it more polite than just ない. I wonder if something is changing with the passage of time, to where it is becoming more acceptable? I imagine that Hideki, Hagiwara, and Junko ok'd the use of ないです. All three are native to Japan. (I am not certain of the other two contributors.) I use "imagine" though because there is a chance that they worked on different sentences and areas. :)
ないです is acceptable, and your teacher simply wants to add some politeness with ない form. Now most of (nearly 100%) Japanese thinks it correct and uses ないです daily. But you guys should know, this form is thought to be not "correct", and a little "newer" style. ありません is the most grammatically correct form if you want to add politeness. But I think this phrase sounds something like "too polite" if I hear in conversations.
元気ではありません。 True polite form
元気ではないです。 More contemporary
元気じゃない。 More colloquial
元気じゃありません。 colloquial, with politeness
元気じゃないです。 colloquial, contemporary, with politeness
Certainly, your sentences are NOT "wrong" for want of a です. This should be corrected.
A more interesting question -- and probably beyond the scope of what can be taught with Duolingo -- is whether in a real-world situation, a Japanese person would end a sentence with just "大きくない”, or else add a particle like "よ" , "ね" , "さ"、”わ" or something similar to shade the meaning（"大きくないよ！” ー "It's not big, I assure you"; "大きくないね" -- "It's not big, don't you agree?"; or "大きくないぞ" -- "It's not big, ya wimp!!")