Das ist eine unsere Stärken vs unserer Stärken

Question

Das ist eine unsere Stärken.
Das ist eine unserer Stärken.

Which is correct? And why?

Can someone please explain it to me (in English :)

Thanks in advance!

Answer 1

Differently said, and shorter ;-) :

The sentence "Dies ist eine unserer Stärken" is in fact the shortened version of: "Dies ist eine Stärke unserer Stärken."

Now, let's say the long version with a preposition: "Dies ist eine Stärke von (+ D.) unseren Stärken." -> Dies ist eine von unseren Stärken.

Let us now replace "von (+ D.) unseren Stärken" by a genitive, and we get: "Dies ist eine unserer Stärken".

Your problem is that in "Dies ist eine *unsere Stärken" we would either have three nominative forms (dies; eine; unsere Stärken) or a two nominatives (dies; eine) + an accusative (unsere Stärken). Neither of these constructions exists, grammatically and logically, as this one strength is part of several strengths. That's what we call genitivus partitivus, and that's it. :-))

Answer 2

This is correct:

Das ist eine unserer Stärken.
This is one of our strengths.

The part »unserer Stärken« (of our strengths) is in genitive case. This grammatical case is used for many circumstances, one of them is for possessive objects, i.e for objects that express some form of ownership. Here we are talking about one specific strength, that belongs to (is owned by) a group of other strengths (which again belongs to a person (i.e. to us) but this kind of ownership is not important for the choice of genitive case).

If something is part of something else, and when you express this ownership with genitive case, then you call this specific usage of genitive case:

• genitivus partitivus = Genitive case for a group of which something else is a part.

  Michael ist der Leiter der Gruppe. Michael is the leader of the group.
  Walter hat zwei der Gemälde verkauft. Walter has sold two of the paintings.
  Du kannst nur mit einer meiner Töchter ausgehen. You can only date one of my daughters.

So, now it should be clear, that you have to use genitive case here.

Most objects are not just one word, but a small group of word, one of them a noun. Here the object consists of two words:

• a form of the possessive pronoun »unser«
• a form of the feminine noun »die Stärke«

This group must be transformed into plural and into genitive case.

If you are unsure, you can look up inflection tables:

• https://de.wiktionary.org/wiki/Stärke
  On this site you will find:
  Genitive plural of »die Stärke« is »der Stärken«.

• https://de.wiktionary.org/wiki/unser
  There are two homonyms of »unser«. One of them is a personal pronoun, but this is not what we we are talking about here. To see the other one (the possessive pronoun) you need to scroll down. And then you will see:
  Feminine genitive plural of »unser« is »unserer«.
  There is also the form »unsrer«, but is a rare. You use it in poems and lyrics when you need a word with two syllables (uns-rer). The standard form is »unserer« which has three syllables (uns-er-er).

So, the genitive object must be:

unserer Stärken (standard form)
unsrer Stärken (alternative form)

And the complete sentence:

Das ist eine unserer Stärken. (standard)
Das ist eine unsrer Stärken. (sometimes in lyrics, poems, etc.)