Learn Icelandic: Eru þær mjög glaðar? - Are they feminine very glad?

In Icelandic, the word for "they" changes depending on the gender of the people or things you are talking about. þær is used when talking exclusively about females (or grammatically feminine nouns). If the group was all male, you'd use þeir, and for a mixed group of males and females, you'd use þau.

Why does the word for "glad" change to glaðar?

Adjectives in Icelandic have to match the gender and number of the noun or pronoun they describe. Because þær is feminine and plural, the dictionary form glaður (glad/happy) changes to its feminine plural form, glaðar.

Why does the sentence start with Eru?

To make a yes/no question in Icelandic, you simply swap the order of the subject and the verb, just like in English. The statement Þær eru mjög glaðar (They are very glad) becomes Eru þær mjög glaðar? (Are they very glad?).

If we were asking about a mixed group of men and women, how would this exact sentence change?

You would need to use the neuter plural pronoun þau for a mixed group. Because the pronoun changes to neuter plural, the adjective must change to its neuter plural form too, making the sentence: Eru þau mjög glöð?