Tom was nowhere to be found.His teacher looked everywhere for him but in vain.
问题描述：
Tom was nowhere to be found.His teacher looked everywhere for him but in vain.
这句子是个改错题,看不出哪里错了,答案把but in vain.前面加了个“,”变成Tom was nowhere to be found.His teacher looked everywhere for him,but in vain.
答
but连接两个并列的句子,完整说法是：His teacher looked for him everywhere,but he(the teacher) was in vain.