To express (just) in case in French, you use the expression (juste) au cas où.
How to use "au cas où" with Mode conditionnel in French
Look at the following examples:
J'en ai racheté, juste au cas où.I bought some more, just in case.
Nous avons acheté de la crème solaire, au cas où.We bought some suntan lotion, just in case.
Au cas où vous vous demanderiez où elle est, elle est allée au marché.In case you're wondering where she is, she went to the market.
Je te le dis au cas où tu ne serais pas au courant.I'm telling you in case you weren't aware of it.
The delicate point here is the question of the mood following au cas où. Indeed, as it expresses an uncertain state or an hypothesis, you could be tempted to use Le Subjonctif. That would be a mistake!
With the expression (juste) au cas où, you can only use Le Conditionnel.
Je te le dis au cas où tu ne le saurais pas.I'm telling you in case you don't know.
Au cas où vous auriez faim, je vous ai fait un sandwich.In case you get hungry, I made you a sandwich.
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