核心问题
How come he isn't here?
如何分析 how come?
词典解答
how come 引导的句子属于不规则的疑问句。主要用于美国英语，尤其是非正式语体。请注意，这种句子虽然是疑问句，但是主谓语的语序却采用陈述句的语序。
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How come you are late? (DAI) 你为什么迟到?
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How come they left you in the dark? (LDCE) 他们怎么把你蒙在鼓里了?
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How come that a lion was wandering about the streets of Rome? 一头狮子怎么会在罗马的大街上到处溜达？
[附注]how come 是 how does(did) it come that 的省略的形式。这就解释了为什么 how come 之后的主谓结构采用陈述句语序，而不采用疑问句语序。