undanfarna

In a sentence

• Ég hef verið að lesa mikið undanfarna daga.

  I have been reading a lot during the past few days.

  Here 'undanfarna' is used as a descriptor to denote the recent days in plural, modifying the noun 'daga'.

• Húsbyggingin hefur verið í miklum framförum undanfarna mánuði.

  The construction of the house has seen much progress over the past months.

  In this sentence, 'undanfarna' describes the recent months, again in plural form, linked to the noun 'mánuði'.

• Ég hef ekki sofið vel undanfarna viku.

  I haven't slept well over the past week.

  'Undanfarna' here is applied to a feminine singular noun 'viku' in accusative case, showing the recent time frame.