Learn Icelandic: Þau eru fræg vegna svona myndbanda. - They are famous because of such videos.

Svona (meaning "such" or "like this") is an invariable word. This means it never changes its shape to match the gender, number, or case of the noun it describes. Even though myndbanda is in the genitive plural, svona stays exactly the same.

Why is the word for videos in the form myndbanda?

The preposition vegna (because of) always forces the noun that follows it into the genitive case. The base word is the neuter noun myndband (video), and its genitive plural form is myndbanda.

Why is the adjective for famous written as fræg instead of frægir?

The adjective must match the pronoun it describes. Since the sentence uses the neuter plural pronoun þau, the adjective frægur (famous) takes its neuter plural form, which is fræg. If the sentence were about a group of men (þeir), it would be frægir.

When do we use þau for "they" instead of þeir or þær?

Þau is the neuter plural form of "they". You use it when referring to a mixed group containing both males and females, or when replacing plural neuter nouns.