老师走进来的时候，他们为什么在笑？
Why were they laughing when the teacher walked in?
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were英 /wɜː/|美 /wər/
laughing英 /ˈlɑːfɪŋ/|美 /ˈlæfɪŋ/
解释
特殊疑问句：why + were + 主语 + laughing，询问过去某时刻正在笑的原因。
Why were they laughing when the teacher walked in?
特殊疑问句：why + were + 主语 + laughing，询问过去某时刻正在笑的原因。