Learn Russian: Студент вас внимательно слушает. - The student is listening to you attentively.

вас is the Accusative case form of the pronoun вы (you). We use the Accusative case for the direct object—the person or thing receiving the action. Since the student is listening to you, "you" are receiving the action, so вы changes to вас.

In English we say "listening to you." Where is the word for "to" here?

The Russian verb слушать (to listen) doesn't need a preposition. It takes a direct object right away. So, instead of saying "listen to you," in Russian you simply say "listen you" (слушает вас).

Why is вас placed before the verb instead of after it?

Russian word order is much more flexible than English because the word forms (like the change from вы to вас) show who is doing what. While you could say Студент слушает вас внимательно, putting the pronoun before the verb is a very common and natural way to structure sentences in Russian.

Why does внимательно end with an -о?

Words that describe how an action is done (called adverbs) often end in -о in Russian. Just like adding "-ly" in English changes "attentive" to "attentively," changing the ending of the adjective внимательный (attentive) to -о gives us the adverb внимательно.