тогда́
adverb
in various senses
then; at that time
тогда́-то и ну́жно бу́дет э́то сде́лать
it is then that it must be done, that will be the time to do it
когда́ он отказа́лся, тогда́ я реши́л де́йствовать
when he refused I decided to act, it was when he refused that I decided to act
then (at that time)
then (in that case, only then)