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

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.

By 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…then

• If nobody is willing to help, then I’ll have to do it myself.