Learn Icelandic: Þú ert nemandi.

In Icelandic, the verb vera (to be) changes depending on who is doing the action. Because the subject is þú (you), we must use the matching form ert (are). If the subject were ég (I) or hann (he), we would use er.

Why isn't there a word for "a" before nemandi?

Icelandic does not have an indefinite article (the words "a" or "an" in English). You just say the noun directly, so nemandi translates perfectly to "a student" in this sentence.

How do I pronounce the first letter in Þú?

The letter Þ (or lowercase þ) is unique to the Icelandic alphabet and is called "thorn." It is pronounced exactly like the "th" sound in the English words "thought" or "thumb."

Can I use þú when talking to a whole classroom of students?

No, þú is strictly singular, meaning it is only used when you are talking to exactly one person. When addressing a group, you will use a different word for the plural "you" later on.