幸亏...不然/否则 means "fortunately X happened, otherwise Y would have happened." It expresses relief about avoiding a bad outcome.
Expressing relief - Lucky that X happened
Counterfactual - Otherwise Y (bad) would have happened
Gratitude - Thankful for the fortunate circumstance
Fortunately you reminded me, otherwise I would have forgotten.
不然 is more colloquial, 否则 is more formal. Both work in this pattern.
幸亏 vs 多亏
- 幸亏 = fortunately (general luck)
- 多亏 = thanks to (credits someone/something)
不然 and 否则 are interchangeable
- 不然 is more colloquial
- 否则 is more written/formal
The second clause is often hypothetical negative
Fortunately you reminded me, otherwise I would have forgotten.
Fortunately I brought an umbrella, otherwise I would have gotten wet.
Fortunately it was discovered early, otherwise the consequences would have been serious.
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