万一...就 is used for unlikely but possible scenarios. It expresses "just in case" or "what if" something happens.
Contingency planning - Preparing for unlikely events
Expressing worry - What if something bad happens?
Giving advice - Take precautions just in case
In case it rains, then bring an umbrella.
万一 implies the situation is unlikely (less than 50% chance). For likely situations, use 如果.
万一 is for UNLIKELY situations
- ✅ 万一堵车 (traffic jam is possible but not expected)
- ❌ 万一太阳升起 (the sun rising is certain)
就 can sometimes be omitted in casual speech
Often used to give cautionary advice
In case it rains, then bring an umbrella.
What if he does not come?
In case something goes wrong, then call me.
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