If this phrase is said by phone or messenger, it means that the person is going to sleep and does not plan to talk. Game over. This is a soft way to refuse from boring conversation. If you are an interesting person, you will not hear it.
A very similar phrase "Я собираюсь спать". If you hear this one, you can speak.
кость (f) – a bone, мышь (f) – a mouse
The trick is to look at the examples in a declension table (in this case, feminine words in the third declension). Wiki has a well-maintained table that you can bookmark and check whenever this sorta question crops up: https://en.m.wikipedia.org/wiki/Russian_declension
to add an article at all is quite odd in English. To be in bed would indicate that I'm in my bed or a bed in a place where I'm staying such as a hotel. To be in a bed could technically mean any bed anywhere and wouldn't necessarily mean for the purpose of sleeping or, well you know, something else you might do in bed... :D
Well, the sentence in russian means that you're in bed to go to sleep (usually), like when your mom tells you "go to sleep" and you reply from your room "I'm already in bed", that's the case where this phrase is used the most, so it might be so that english translation with an indefinite article is incorrect because it doesn't mean this precise situation. But language is a very flexible thing :3
"a" is used to mean any of that object
"the" is used to mean a specific one of that object
In this case you use neither because because it implies your own bed and the other person would know this. "I am already in my bed" is also okay but the possessive is unnecessary because why would you be in another persons bed?
"On the bed" would use the preposition на instead of в. When you are on the bed, you could be sitting, lying, standing, but does not imply that you will be sleeping any time soon. When you are "in bed", that means that you are likely under the sheets/blankets or at least lying down ready to sleep.
Unrelated coincidence. Blood comes from Balto-Slavic roots, leading back to proto-indo-european *krewh₂- (~krevh), where we actually get the word "cruel" from.
Bed comes from a Latin loanword grabātus (meaning bed/mattress/cot) from Greek κράββατος (krabbatos, meaning mattress/pallet), which in turn came from Ancient Macedonian *γράβος (grabos), meaning oak, the type of wood.
Interestingly enough, граб is still used to refer to a type of birch tree, the hornbeam tree, from those same original roots.
Well, I am Polish, not Russian but I'm pretty sure that на=on and в=in in Russian as well. So if "in bed" was accepted as a translation of "на кровати", then I'd say it was a mistake on Duo's part. The reason might be that being "in bed" usually means being on top of the bed anyway, so maybe the moderators accepted it as close enough. Which is probably going to confuse people because the same doesn't apply to other nouns, eg. "in the building" vs "on the building".
It is a construct in English. In certain sentences you can omit the article to pass some additional implicit information - "I am in bed" not only communicates my physical location - it also means that I am intending to go to sleep.
"I am in prison" - in addition to what it means literally, it also implies that I'm a convict; "I am in the prison", on the other hand, doesn't imply anything - I could be there for any reason, e.g. visiting someone
"I am in school" - it implies that I'm a student
"I am in hospital" - it implies that I'm being hospitalized
As Russian doesn't have articles, you can't deduce this stuff just from the sentence, so additional explanation will often be required to avoid misunderstandings :-)
It is a correct sentence, but for this exercise you don't need "my". A possessive pronoun is not in the Russian sentence.
Is it that this sentence is more literal than the English, "I already went to bed"?
That is, in English "went to bed" is more like decided to go to sleep than going to a literal bed. For example, one might "go to bed" on a couch.
Or should "I already went to bed" be an accepted answer?