他老是记不起把自己阅读时用的放大镜放在什么地方，所以助手们会随身携带备用的。He misplaces his reading glasses with such regularity that aides carry extras.大货车载着备用的弹药。The wagons carried spare ammunition.我可能需要一套备用的衣服。I might need a spare set of clothes....

“备用的”的英语可以翻译为：eady，spare，[计] off-duty，[化] stand-by ...