途端

1. Origins, structure, and basic meaning

• Structure: The spelling is 途端, read as とたん. It is a noun-like expression following a verb in the た-form to mark an unexpected event occurring immediately after the first action.
• Basic usage: Marking the instant after stepping outside appears in 外へ出た途端, while becoming sleepy immediately after sitting appears in 座った途端に眠くなる.

2. Common pitfalls and similar expressions

• Difference from 時
  • It conveys both immediate sequence and an abrupt or unexpected change, rather than simply identifying a time.