Back then is an expression you can use in order to refer to a specific period of time in the past. For example, if you are teaching about Middle Age, you could say "back then, people used to have slaves to work for them". Some synonyms are: At that time, In those days.

"I believe people had more respect for teachers back then". To use "back then", the listener must know which period in the past your are refering to FIRST -- before you can use the above sentence. Example>I was born in the 60s . I believe that people had more respect for teachers back then. ( back then = back in time during the 60s) > We met 25 years ago . We were still students back then. If you say the above without telling what you mean by "THEN", the reader will have to ask you this : : "Back then ? When is "THEN"?" If you are not refering to any particular time in past , you can say any of these : "I believe people used to have more respect for teachers " "I believe people had more respect for teachers in the past".