How atypical/strange would it be to say für in this sentence?
i.e. Sie leben für achtzehn Monate
When is it "Monaten"? Is it when we have accusative and dative? And is it a weak noun?
Wikipedia says: ʿNouns in plural that do not already end in -n or -s (the latter mostly found in loanwords) gain an -n in the dative case. e.g. der Berg, die Berge, den Bergen.ʾ
https://en.m.wikipedia.org/wiki/German_declension#Nouns
I would like to know this too
Only if it is plural dative
https://de.m.wiktionary.org/wiki/Monat
"Monaten" when it's plural preceded by a dative preposition; e.g., Ich wohne seit drei Monaten hier. And AFAIK, it's not a weak noun.
Can someone explain why the sentence is in this way? Why für is missing?
Why not 'Wohnen'. We don't know the context of this sentance.
No. The English sentence doesn't mean "they reside for eighteen months."
Could this sentence be used for talking about someone with a disease, to say they only have eighteen months to live?
You'd say that the same way in German: "Sie haben nur noch 18 Monate zu leben."
Sie leben 18 Monate LANG should also be correct
Why not Achtzehn monate leben sie ?