The word then can be used as an adverb and a conjunction. Then means at that time. It can refer to both past and future. Note that then is not used to refer to the present.
- We were living in France then. (= At that time we were living in France.)
- I was working at a pub then. (= At that time I was working at a pub.)
- Will you still be here then? (= Will you still be here at that time?)
- I will be back in two hours. Will you be able to wait until then?
From then means from that time onwards.
Since then means between that time and now.
- Bombay was once a sleepy fishing town, but things have changed a lot since then.
- She had finished the report by then.
As an adjective then can be used to refer to a person who had a particular job at a particular time in the past.
- The then Prime Minister of India was Indira Gandhi.
Then can also mean next or after that.
- We will go to Paris first, then to Rome.
- He glanced quickly at her and then looked away again.
Then can mean ‘in that case’.
- You say you are ill; then why don’t you go home and get some rest?
Then can be used for saying what the result must be if a particular condition is true.
- ‘She won’t pay the dues.’ ‘Then we won’t renew her membership.’
- If nobody is willing to help, then I’ll have to do it myself.