- 改札口を出たとたんに、傘を電車の中に忘れたことに気づいた。As soon as I got out of the ticket gate, I realized that I had forgotten my umbrella on the train.
- 三日ぶりに会った幼い娘は、私の顔を見た途端、わあーと泣き出した。Three days later, when my young daughter saw me, she burst into tears.
- 火災報知器のベルが鳴ったとたんに、みんな慌てて外に飛び出した。As soon as the fire alarm bell rang, everyone rushed out.
As soon as the first thing happened, the following thing happened. The latter thing is mostly unexpected, and it is something that has already happened that the speaker personally experienced, so the end of the sentence can only be in the past tense.