Besides just memorizing the point to keep in mind for word like ethnos, genos etc. is that there used to be a sigma that dropped out (as sigma between vowels usually does), so the actual forms are contracted ethnesos -> ethnous, ethnesa -> ethnea -> ethnê. In some archaic dialects uncontracted forms are retained, e.g. "algea" (pains) in the Odyssey prologue.
Besides just memorizing the point to keep in mind for word like ethnos, genos etc. is that there used to be a sigma that dropped out (as sigma between vowels usually does), so the actual forms are contracted ethnesos -> ethnous, ethnesa -> ethnea -> ethnê. In some archaic dialects uncontracted forms are retained, e.g. "algea" (pains) in the Odyssey prologue.
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