No, he means "nominative". In "he is one of us" and "er ist einer von uns", the verbs "is/are/am/be/being/been/was/were" and "sein/bin/bist/ist/sei/war/..." take a predicate nominative as their complement rather than an accusative or dative complement that most verbs take. That's why people often correct "it's *me/him/her/us/them" to "it's I/he/she/we/they." In this case, "einer" is a masculine nominative, like "der".
Mizinamo's CanooNet link is broken as of December 2019. I think the page has been moved to here:
In summary, einer is an indefinite pronoun and can be used as a substitute for a noun. It is declined like the definite article and other der- words such as dieser. Other indefinite pronouns we have seen already include keiner and jeder.
As always, I am not a native speaker of German, only an enthusiastic student, so if I've got any of this wrong, please correct me.
einem is dative case, but "to be" takes a predicate in the nominative case, not the dative case.
So it's er ist einer von uns with einer in the nominative case.
(Note that the pronoun einer - which stands alone - has an ending in the masculine nominative, unlike the indefinite article - which stands before a noun - which has no ending there, e.g. ein Mann.)
Guys, this is the exact, short and diffinive explanation to why it' einER and NOT ein or einEM. it's just enough of clarification for me (at this noobi) level. many thanks mizinamo. then i underestand it as such, that in "Er ist einer von uns". becuae of theb verb "ist", ein takes a predicate in the nominative case, it predicts masulin for Er ans is forced into "Er" ending. it's more beautiful and rythmic by the way. corret me if i am wrong.
Because it stands alone here, without a noun after it -- it's acting as a pronoun, not as an indefinite article.
Compare, for example, Tom ist ein Mann und Paul ist auch einer. "Tom is a man and Paul is one, too". The first one is ein because it's in front of Mann; the second one is einer because it's standing alone.
Possessive adjectives and possessive pronouns have this difference, too, for example Hier ist mein Pferd und hier ist deins. "Here is my horse and here is yours." -- the second one is not dein as in dein Pferd (possessive adjective before a noun), but deins with a neuter nominative ending -s (possessive pronoun standing alone).
You'll note that English does this for possessives as well, distinguishing between "my pen and yours" or "your pen and mine", rather than "my pen and your" or "your pen and my" or "mine pen and yours".
No, not possessive in this context. It is nominative, because the sein-verb "ist" has no direct object.
Think of the words "ein" and "Einer" as two separate words. Use "ein" as the possessive "a/an" and use "Einer" as the noun "One".
Another angle on this example would be if it were negated. In English: He is NOT one of us. Now, in German you could simply negate Einer with "nicht" placed in front of it, but that sounds awkward. It is more grammatically correct, and sounds better to say: Er ist keiner von uns.
Tl;dr: "Ein" --> "a/an" "Einer" --> "one" (use it as a noun but don't capitalize)
No, because "uns" is not possessive. I'm thinking it would have to be "Er ist einer von unserem" but my grasp of dative prepositions is tenuous so I could be wrong
These declension issues come up time and time again and despite the incredible patience and generosity of the mods, it’s clear the Duolingo didactic is not well suited to address this topic in a stand alone manner.
There is an “all in one” declension chart at https://germanwithlaura.com/declension/#all-in-one-chart that covers: Definite Articles (der, die, das, etc.) Indefinite Articles (ein, eine etc.) Demonstratives / der-words (dies-, jed-, etc.) Indefinite Pronouns Relative Pronouns (der, dessen, etc.) Possessive Pronouns Possessive Adjectives (mein, dein, etc.) Strong Adjectives (No Determiner) Weak Adjectives (With Determiner) Mixed Adjectives (with ein-word Determiners)
You have to learn the basic rules of the chart which is not difficult and could be worth a look for anyone who is finding this topic descending into an impenetrable mental fog.
So, if "He is one of us" translates to "Er ist einer von uns", then would the female or neuter subject be something like these:
"She is one of us." = "Sie ist eine von uns."?
"The horse is one of us." = "Das Pferd ist eines von uns."?
Is that right or entirely incorrect?
einer is not dative. It's masculine nominative singular.
It's not followed by a noun -- it's a pronoun standing on its own -- and so the form is different from that of the indefinite article.
For example: Das ist ein Mann und das ich auch einer. "That is a man and that is one, too."
The first one is ein because it's before Mann; the second one is einer because it stands by itself.