I wondered what the twins who were in a bad mood were doing yesterday afternoon? 语法上，在干什么，是典型的进行时，必须用be doing。。你只有doing漏了个系动词be。 若想再精益求精一点，那么，你的中文其实有歧义。到底是“昨天下午那对心情不好的双胞胎”，还是“那对心情不抗露好的双胞胎”昨天下午在干什么。 如果是前者，则应该是： ...what the twins （who were）抹抽遮 in a bad mood yesterday afternoon were doing. 如果是后者，则是： ...what the twins （who were） in a bad mood were doing yesterday afternoon. 这惑绣两句话里括号里的部分都可以省略。语法完全合格。