How am I supposed to tell whether it's teacher or teachers?
Der Lehrer is "the teacher" singular. Die Lehrer is "the teachers" plural. The article changes from der to die when it becomes plural. Der Lehrer means the male teacher. Die Lehrer means the teachers (any or all genders). Die Lehrerin means the female teacher. I'm guessing that a group of female teachers would still be die Lehrer.