Learn Icelandic: Þau hafa ekki fundið fræga minnisvarðann. - They have not found the famous monument.

This is the supine (sagnbót) form, used with hafa to say someone "has done" something. Finna is a strong verb, meaning it forms its supine with the ending -ið instead of a consonant like -t. It also has a vowel change from i in the infinitive to u in the supine, a common pattern for strong verbs.

Why is the adjective fræga (famous) used instead of frægan? Isn't it masculine accusative?

Yes, it is masculine accusative! However, because the noun has the definite article attached (minnisvarðann meaning "the monument"), the adjective must use the weak declension. For a masculine accusative noun, the weak adjective ending is just -a, making it fræga instead of the strong form frægan.

How do we get the form minnisvarðann from the base word minnisvarði (monument)?

The verb finna takes an object in the accusative case. Minnisvarði is a weak masculine noun, so it changes to minnisvarða in the accusative. When we attach the definite article meaning "the", we add -nn to the end, resulting in minnisvarðann.

Why use Þau for "They" instead of Þeir or Þær?

Þeir is strictly for a group of males, and Þær is for a group of females. Þau is the neuter plural pronoun, which is used as the default for a mixed-gender group or when the gender of the people is unknown.