Questions & Answers about Þær eru ungar og fallegar.
Why does this sentence use þær for "they" instead of þeir or þau?
In Icelandic, the word for "they" changes depending on who or what you are talking about. Þær is specifically for a group of females (or feminine nouns). If the group were all male, you would use þeir, and for a mixed-gender group, you use þau.
Why do the adjectives end in -ar here (ungar, fallegar)?
Adjectives in Icelandic change their endings to match the gender and number of the pronoun they describe. Because þær is a feminine plural pronoun, the adjectives must take the feminine plural ending -ar.
What are the base dictionary forms for ungar and fallegar?
What if this sentence was about a mixed group of men and women?
You would change the pronoun to the mixed-group "they", which is þau. The adjectives would also change to match the neuter plural form, which happens to drop the endings entirely for these specific words: Þau eru ung og falleg.