Breakdown of Þau hlusta alltaf á hvort annað.
þau
they (n)
alltaf
always
hlusta
to listen
á
to/at (with verbs of perception: hlusta á, horfa á)
Questions & Answers about Þau hlusta alltaf á hvort annað.
Why is the phrase hvort annað used here instead of the dictionary form hvor annar?
The phrase changes to match the subject and the sentence structure. Because the subject þau is neuter, both words take the neuter singular form. The first word, hvort, is in the nominative case to match the subject. The second word, annað, is in the accusative case because it is the object of the preposition á.
Since þau means "they" (plural), why are hvort and annað both in the singular?
Think of the phrase literally as "each [one] listens to the other." Even though the subject is plural, the reciprocal phrase breaks the action down to the individual level, so the words are grammatically singular.
Does this specific phrase tell us how many people are listening to each other?
Yes! Forms of hvor annar are used strictly when exactly two people or things are interacting. If three or more people were involved, you would use a form of hver annar instead (which would be hvert annað in this sentence).
Why is alltaf placed where it is in the sentence?
In a standard main clause, adverbs like alltaf (always) usually come immediately after the finite verb (hlusta) and before the prepositional phrase (á hvort annað).