Differences in noun constructions - Help!

Hi Norwegian natives,

Could someone kindly explain in simple terms the difference between a and b for the following two “noun construction” examples below.

  1. a) “De søteste tvillingene” and b) “de søteste tvillinger”
  2. a) “Den siste dagen” and b) “Den siste dag”

Thanks in advance for your help.

Best Regards,
Andy

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