Learn Icelandic: Kennarinn sér okkur.

Okkur means "us". It is the accusative (object) form of the pronoun við (we). Because the teacher is doing the seeing, the teacher is the subject and we are the object. The verb sjá (to see) puts its object in the accusative case, so við becomes okkur.

Where does the verb sér come from?

Sér is the "he/she/it" form of the verb sjá (to see). Sjá is an irregular verb, which is why the vowel changes from "á" to "é" when we say "the teacher sees" (kennarinn sér).

How do we know kennarinn means "the teacher" and not just "a teacher"?

In Icelandic, the word for "the" is usually added to the end of the noun rather than being a separate word in front of it. The base word is kennari (a teacher), and the -nn attached at the end makes it kennarinn (the teacher).