"Verk" is the accusative or the dative of verkur?

Well. My problem is that I don't understand if vera með is + accusative or + dative XD
In a (free) textbook I found "Stína er með löngu nefi.", here nefi is a dative, right?
I saw many vera með + accusative in Tatoeba, but obviously I trust more you than my textbook. But is its sentence good too?

Usually, "vera með" takes accusative when it refers to a thing you have. But you can also use "vera með" to mean "be with", as in: Pharamp er með Trang (niðri í bæ); where it takes the dative.

I don't recall ever hearing the form your textbook uses. It could be an archaic form, but I can't tell for sure. I'll see if I can find out. Is that an on line textbook?

Oh great! I studied it this morning.
My textbook is this one
http://www.amazon.co.uk/Referen.../dp/1897999461
It's a quite bad and ugly book, unfortunately full of errors. But it was the only Icelandic grammar in the library of my city (and that's a very big library!), so I took it anyway. I'm studying mainly online :/ with IcelandicOnline, its dictionary and Tatoeba...

Ah, I think I've found this sentence in another textbook.
This one: www.giuristipernaso.it/KENNSLUB.pdf
It's an Italian-based grammar :)

Anyway, "Stína er með löngu nefi." should be... nef... but I don't find the accusative for langur :/ How is the correct sentence?

That book looks suspicious. The author, George Carcas, has similar grammar guides for a number of languages; Slovenian, Ukrainian, Belorussian, Slovakian, Macedonian, Afrikaans and Icelandic. The absence of books on other Nordic languages along with the strange grammar (and seeing how you say it has many errors) makes me skeptical that the author is sufficiently proficient in Icelandic to write a book on the language.

I had a quick look at the PDF textbook you found. I'm not sure about that one either. The author has a very good command of the Icelandic language, but still makes few mistakes in the free-flowing text. That doesn't, however, imply that there are inaccuracies in the lessons.

One quick note on the setup of that book: In Icelandic, it's customary to list the tenses in the order: nominative, accusative, dative and genitive (nefnifall, þolfall, þágufall og eignarfall). A minor issue, but just so that you know, because when asking Icelanders, they will always reply with the word forms in that order.

As I'm about to do:

The adjective is "löng" in the feminine singular (considered the "base" form for adjectives). The declination is:

karlkyn: langur, langan, löngum, langs
kvenkyn: löng, langa, langri, langrar
hvorukyn: langt, langt, löngu, langs

The word "nef" is neuter so we pick the neuter, accusative form: "langt", making the sentence:

"Stína er með langt nef."

Hope that helped.

Yes! It helped incredibly.
For the tense order, even if it's totally inaccurate, I imagine the author used the same order as Latin (that more or less everyone here in Italy studies): nominative, genitive, dative, accusative, (vocative) and (ablative) (Wiktionary is wrong about that :P).

If you know a good method to export from a .csv file only one side of all cards (so you don't get annoyed by my Italian), I can show you my deck (it's only ~500 sentences) ^^ only if you have time! Takk!