They should do it at once.
问题描述：
They should do it at once.
It should be done at once.
答
1）他们应该马上去做它.should有 “应该,竟然,按理说”的意思,陈述句作为情态动词,常做应该意思,后街动词原形.
下面的是：”它应该立刻被做“,be+动词的过去分词构成被动语态,此句中“be done”为被动.愿意为你效劳,