Learn Hardcore Russian: Мой друг рассказывает о закате.

Because the preposition о requires the noun to be in the prepositional case. In this context, закат (sunset) becomes закате to indicate the topic that your friend is talking about.

How do the subject and its modifier work in Мой друг?

Мой друг means "my friend," where друг is in the nominative case as the subject of the sentence, and мой is a possessive adjective that agrees with друг in gender (masculine), number (singular), and case (nominative). This agreement is essential in Russian to maintain grammatical consistency.

Why is the preposition о used with закате, and what does it tell us about the verb рассказывать?

The preposition о means "about" and is used to introduce the topic of conversation. In Russian, verbs like рассказывать ("to tell" or "to recount") typically govern a prepositional phrase to specify what is being discussed. Thus, о закате indicates that the friend is talking about the sunset.

What tense and aspect is conveyed by the verb рассказывает?

Рассказывает is in the present tense and has an imperfective aspect. This form is used to describe actions that are currently taking place or are habitual, implying that the friend is in the process of telling or habitually tells about the sunset rather than having completed the action.