He could have taken a spare key.

他原本可以带上一把备用钥匙的。

《柯林斯英汉双解大词典》

If possible keep a spare pair of glasses accessible in case your main pair is broken or lost.

如有可能留一副备用眼镜以防你常用的那副打碎或丢失。

《柯林斯英汉双解大词典》

I haven't had a spare moment this morning.

我一上午一会儿也没闲着。

《牛津词典》

The U.S.military is buying more than 6,000 of the M-ATVs, at a cost of nearly a million and a half dollars each, including all their equipment, spare parts and transportation to Afghanistan.

VOA: standard.2009.11.02

And that got me to think gosh, these people have so little and I was a student, and I didn't have a lot of money, but I still had something to spare.

看到这张照片，我想,天啊，这些人太贫困了,虽然我是个学生，我没有很多钱，但我还是能够捐出些什么的。

普林斯顿公开课 - 人性课程节选

But as you get older and you have a lot of spare time, you think about what you are doing, which is something I have the luxury of doing right now, and I realized this is more tricky.

但随着你们年龄的增长,有更多空闲时间之后,你就会思考你做的事情,我现在就有很多闲暇思考这些问题,我意识到这东西其实很精妙

耶鲁公开课 - 基础物理课程节选

Hyeonduk Seunim gestured us into a spare, empty room with a low table at the centre.

BBC: A free meal with South Korea's monks

They are constantly on their phones trying to find anyone who has a spare ticket.

BBC: Sachin Tendulkar's big 40th birthday 'party'

Why penalise claimants who have no alternative but to live in accommodation with a spare bedroom?

BBC: PCC Stephen Bett fears welfare changes will cause crime