1. He like talking with the native speaker in the teacher's room.

Question

2. He likes talking with the native speaker in the teachers' room.

(Which one is correct, 'teacher's' or 'teachers''?)

Answer 1

Number 2 is correct because teachers' is the plural possessive. The teachers' room belongs to all of the teachers.

Of course, this could mean the classroom of one particular teacher - in which case it would be teacher's room (singular possessive).