一旦

1. Origins, structure, and basic meaning

• Structure: 一旦 is read as いったん and is an adverb meaning either temporarily or once a condition has come into effect.
• Basic usage: Use 一旦中止する for a temporary pause and 一旦出ると for a consequence that follows once one leaves.

2. Common pitfalls and similar expressions

• Distinguishing the two senses
  • Decide between temporarily and once by reading the following verb and conditional construction together.
• A condition hard to reverse
  • 一旦～と often suggests that once a stage is reached, its consequence is difficult to avoid or undo.