As soon as
This structure indicates that something happens immediately after another action is completed.
Examples
ドアを開けたとたん、犬が飛び込んできた。
As soon as I opened the door, the dog jumped in.
彼は走り出したとたん、転んでしまった。
As soon as he started running, he fell over.
電話がかかってきたとたん、彼女は驚いた。
As soon as the phone rang, she was surprised.
Quick check
ドアを開けた___、犬が飛び込んできた。