～たとたん
the moment that
～たとたん is made from the た-form of a verb + とたん（に）. It means “the moment that” or “the instant that”: as soon as the first action happened, the second event occurred immediately after.
The second event is typically something sudden or unexpected that the speaker did not control like a discovery, a reaction, a change. It is not used for the speaker's own planned next action.
Sudden event right after an action
The clause before とたん sets the trigger and the clause after it reports what happened in that same instant.
ドアを開けたとたん、猫が逃げた。
“The moment I opened the door, the cat ran away.”
- ドア = door
- を = object marker
- 開けたとたん（あけたとたん）= the moment (I) opened
- 猫（ねこ）= cat
- が = subject marker
- 逃げた（にげた）= ran away
立ったとたん、頭が痛くなった。
“The moment I stood up, my head started to hurt.”
- 立ったとたん（たったとたん）= the moment (I) stood up
- 頭（あたま）= head
- が = subject marker
- 痛くなった（いたくなった）= started to hurt
How to form it
Verb た-form + とたん（に）
- 開ける（あける）= to open → 開けたとたん（あけたとたん）= the moment (someone) opened
- 見る（みる）= to see → 見たとたん（みたとたん）= the moment (someone) saw
- 着く（つく）= to arrive → 着いたとたん（ついたとたん）= the moment (someone) arrived
Key idea
～たとたん freezes a single instant: the very second X happened, Y happened. Usually it's something the speaker noticed with surprise rather than something they chose to do.
Other N3 grammar
Updated .