> Why first time shouldn't we put an artikle, but second time, we do?
The first sentence specify an property, f.e. an profession "(S)He is a teacher.". As we use the word "teacher" as an adjective to specify a property of person "(s)he", we will leave the artikle in german.
The second sentence specify an person, f.e "Where is the Teacher?". As we ask there for an person, we use the artikle.