What does a taekwondo instructor say when you enter a class?

Answer 1

This depends on the class. Usually it is some form of "hello / How are you?" In korean this would be:
An nyeung ha say yo - 안녕하세요
OR
An nyeung ha shim ni ka - 안녕 하십 니까

The second is the most formal method, usually used when addressing a Grandmaster.

it should sound like the English words:
on young hah say yo

Answer 2

I don't know if there is an actual thing the instructor says, but usually a greeting, whether in English (Hello) or Korean (ahn-yeung-ha-seh-yo/ahn-yeung).