FYI: this sentence in English is incorrect.
I bought her some books about Mexico in case they were useful for his trip to Acapulco.
"in case they would be useful"
Also could use past conditional, or "subjunctive" past and present although that's not what we call it.
in case they would have been useful
in case they might be useful
in case they might have been useful
We revised the translation. We will change it to "in case they might be useful".
Gracias y saludos.
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