This seem to be in the genitive, instead of the nominative "Спокойная ночь".
Good morning, good afternoon and good evening on the other hand all use the nominative ("Доброе утро", "Добрый день", "Добрый вечер").
Is there a reason for the choice of case in this particular phrase? Or is it just a completely random language quirk?
Well then, that by itself does not solve the mistery of why "желаю" does not take the accusative ("Спокойную ночь" ?) in this example.
However, by googling based on this omitted "желаю" I've found that some verbs like "желать" or "ждать" can be used with accusative OR genitive for the direct object, and it somehow is related to the "vagueness" or "abstractness" of the thing that is being waited / wished.
So, have a Lingot for that very useful pointer!
Yes, I've read that "ждать" and "хотеть" are used normally with the accusative, but they take the genitive if the thing that is "waited" or "wanted" is abstract (like in "waiting for true love" (genitive) vs "waiting for the five o' clock train (accusative) ).
The same source states, as you say, that "желать" indeed takes always the genitive. I mentally associate it with the idea that "wishes are always abstract" :P (Like if you said "I have wishes OF such and such thing").
It is a shortening. Compare:
- Good morning! - Morning!
Good night! - Night!
Доброе утро! - Доброе!
- Спокойной ночи! - Спокойной!
In English, you drop the adjective, and in Russian it is the noun that's dropped :-) The shortened version of "доброе утро" is very common, and it sounds like a confirmation, sort of "indeed, this morning is good". "Спокойной" instead of "Спокойной ночи" is less common.
Note that in Russian, you only use the shortened version when replying to someone's greeting or bedtime words. You can't say just "Доброе!" when you are the first to greet someone in the morning. And, of course, this shortening is optional. It is perfectly fine to always use the full phrase.
The problem is that direct translations while accurate can be misleading.
Duo is saying that while the Russian usage is typical and very common, the English peaceful and calm constructions are not typical.
I can't think of a time when anyone wished me a peaceful or calm night. Nothing wrong with it, just unusual in ordinary circumstances. Totally appropriate if for some reason there is a possibility it won't be calm or peaceful.
I never understand why they say I "used the wrong words" rather than saying I did a typo. I've come back to Duo after not doing it for a year or two, and tried "Спакойне ноче" which was a failure. I know it's wrong, but I don't get why sometimes they say it's a typo and sometimes they don't...
are you having trouble with the the "ы" sound in particular?
to pronounce this: position mouth so that you're saying an "ee" sound, like the "ee" in "key"
now instead of your tongue touching your bottom teeth, move it up, without altering the position of your mouth. your tongue should not touch the sides, roof, or bottom of your mouth. now blow out some air. if it doesn't sounds like something in between "ee" and "oo", you're on the right track. if it doesn't, keep practicing or maybe find a youtube tutorial. or you could just give up. im sure russians will understand what you're saying if you mispronounce one letter