いったん + Verb (たら / ば / と) + clause

[Once ~ / As soon as ~ happens]

Connection: いったん + Verb (たら / ば / と) + clause

Usage: Expresses that once something happens, a certain result inevitably follows and is often difficult to reverse. Emphasizes the irreversibility or strong consequence of the initial action. Common in both formal and informal contexts.

Similar expressions:

• 一度〜たら = once ~ (similar but less emphatic)
• 〜てしまえば = once you do ~ (emphasizes completion)

Examples:

いったん決めたら、最後までやり通すべきだ。(Once you've decided, you should see it through to the end.)

いったん信用を失うと、取り戻すのは難しい。(Once you lose trust, it's hard to regain it.)

この薬はいったん飲み始めたら、途中でやめてはいけない。(Once you start taking this medicine, you must not stop in the middle.)