I'm guessing because it's not a second person verb form grammatically - those often behave differently in other languages.
For example, English just uses the verb with second-person imperatives ("Sing!") but uses a different construction with first-person ones ("Let's sing!"), which includes a pronoun ("us").
Or Marie-Antoinette's supposed phrase "Let them eat cake!" which also used a pronoun ("them") in a sort of third-person imperative.
Koch dir die Nudeln! = Cook the noodles (for yourself)!
That is, you are the person who is benefitted by the cooking -- I am asking you to cook them not for anyone, but for yourself.
Koch die Nudeln selber! = You cook the noodles! / Cook the noodles yourself!
That is, I am saying that nobdy else is going to be doing the cooking, so if you want noodles to be present, it's you who is going to have to do it. (The noodles could be for you or for someone else.)
You can even combine them: Koch dir die Nudeln selber! Indicating that if you want there to be noodles for you to eat, then you will have to cook them, because nobody else will cook the noodles for you.
I don't know what the words like damit, womit are called.
If you are using the Sie form of the imperative the Sie has to be there.
There is, as ever an exception, when you don't write Sie instead you put the verb (Infinitive) at the end of the sentence. This is used for recipes or (less often) with a list of several instructions. I think of this as there being an implied "Sie sollten..." then the instructions. Eg. a recipe to make a pot of tea might be Wasser Kochen. Tee in eine Tekanne einfügen. Kochendes Wasser auf den Tee eingiesen. Fünf minuten ziehen
That's right, the -er is not a grammatical ending here, at least not one related to gender.
Duden ( http://www.duden.de/rechtschreibung/selber_selbst_Pronomen ) calls selber a "particle" and an "indeclinable demonstrative pronoun".
What, for you, is the difference between pasta and noodles?
I suppose you could use Teigwaren for "pasta" in the broader sense, but I think most non-specialists will treat them as one big happy category, whether they're spaghetti or penne or farfalle or whatever: it's all Nudeln.
German does have the word (die) Pasta which means specifically Italian pasta. However, I think it would mostly be seen on Italian restaurant menus, or in discussing Italian pasta dishes or products. The concept of noodles is a very old one in Germany, with a lot of their own types of noodles, so they haven't been quite as quick to adopt the word "pasta" as some other languages, like English. I've seen Nudeln used to describe a wide variety of noodles in German restaurants and cookbooks - including Italian pasta and Asian noodles in restaurants. So while I'm not a native speaker, I'd say translate it as "pasta" only if the original word is Pasta. If it says Nudeln, then translate it as "noodles."
I don't know if Duo accepts "pasta" as a possible translation or not; they might. If that's your only error in the sentence, then it could be the problem. That's just my thinking on general usage in the land and in the language, after 30 years of occasional living and traveling in Germany and Austria.
How would you say "Cook the noodles by themselves", as if you were telling someone to cook the noodles before adding any sauce or spices?
I'd suggest Koch die Nudeln alleine.
Or Koch die Nudeln getrennt (cook the noodles separately).
(Koch die Nudeln alleine could also mean "cook the noodles by yourself" -- i.e. without anyone else being present or helping you. Which is not the same as Koch die Nudeln selber "cook the noodles yourself" -- i.e. rather than delegating the task to someone else or expecting someone to do it for you.)
i would use boil in a recipe or of it is unclear what meathod of cooking is to be used. If the person knows how you you usually cook pasta the you just say cook.
Kochen for english speakers is quite tricky as German has no seperae word for To Boil. The result is that sometimes you do not use Kochen to mean cook you must specify which cooking method will be used. In other situations Kochen works just fine meaning "to cook". You just have to learn this case by case.
If you're speaking of pasta in general, you could say "cook pasta." For example: "When I cook pasta, I always boil it too long."
But for this sentence, which sounds like someone has asked you to cook them some pasta, and you have refused, then you are referring to this specific pasta, and you say, "Cook the pasta yourself, lazybones!" or "Cook it yourself."
Well it's exactly the same word order as in English. In more detail: The sentnce is "imperative" (a command) so the verb comes first, then come the main nouns-here it's the subject "the noodles"- and then come the adverbs or nouns following a preposition-yourself is an adverb in this sentence.
"You cook the noodles yourself."
The "you" is implied enough for this to be accepted, or do you all think not?
"Cook the noodles yourself." means "I want you to cook the noodles. I do not want anyone else to do it. You have to do it, not anybody else. You have to do it yourself."
"Cook yourself the noodles." means "I want you to cook the noodles, and then you can eat them. You are not going to cook the noodles for me, you are going to cook them for yourself."
The German sentence Koch die Nudeln selber. means "Cook the noodles yourself."
"Cook yourself the noodles." would be Koch dir die Nudeln.
Yes, it's wrong.
"Cook the noodles yourself!" (Koch die Nudeln selber!) means that YOU should cook them, not anybody else.
"Cook the noodles by yourself!" (Koch die Nudeln alleine!) means that you should cook them without any help, without anybody else participating in the cooking.
Is "selben" a valid form of "selber"?
Yes and no.
In Duo's sentence, selber is an adverb, which doesn't have any other forms.
But there is an adjective selb- which means "same", which is essentially always used together with the definite article: derselbe Hund "the same dog", dieselbe Katze "the same cat", dasselbe Pferd "the same horse".
But sometimes it's used alone, in which case it takes strong adjective endings, e.g. Selber Preis wie gestern? "Same price as yesterday?" = Derselbe Preis wie gestern? "The same price as yesterday?"
And when the definite article contracts with a preposition, selb- is often written separately as well, e.g. am selben Tag "on the same day" for an demselben Tag.
So selben and selber are both possible forms of the adjective selb- "same".
I imagine it would mean "to yourself".
No. That would be selbst -- also no inflection. Du gibst es dir selbst "You are giving it to yourself."