I was in the midst of trying to educate myself on "collective numerals" when I came across this:
To say that the usage of двое, трое, etc., is limited to the nominative case would be incorrect. Yet, there is no doubt that in many instances the oblique cases of два, три, четыре, etc., are definitely preferred.
From "Table of Collective Numerals," RussianLearn.com.
I am having difficulty understanding the excerpt above and part of the stumbling block is that I am really just guessing at what is meant by "oblique cases." I might still have difficulty understanding the passage, but attempts to comprehend it are stymied by the fact that my notion of "oblique cases" is somewhat nebulous. Are oblique cases all cases but the nominative? Or are they a subset of cases beyond the nominative? And, if so, which ones? I've tried searching for this information online, but have not seen anything yet that addresses this question.