Fact-Checking 'How Comes': Historical Archaism or Outright Grammatical Flub?

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To understand why the trailing "s" causes such intense confusion today, one must look at how "how come" breaks the standard rules of English question formation. In standard questions introduced by interrogative adverbs like "why," English requires subject-auxiliary inversion. You cannot say "Why you went home?"; you must insert the auxiliary "did" and front it: "Why did you go home?"

"How come" flatly rejects this rule. It acts as an entire prefabricated complementizer clause. Because it fundamentally replaces the full proposition "How does it happen that...", the sentence that follows retains normal, affirmative declarative syntax. Consider the contrasts in mechanical execution:

You say, "Why is she upset?" but "How come she is upset?"

You say, "Why did they sell the car?" but "How come they sold the car?"

Because the modern phrase behaves as an invariable idiom rather than an active verb phrase, the word "come" no longer conjugates. It does not shift to "how came" when asking about yesterday's events, nor does it take a third-person singular marker when paired with a singular subject. Saying "How comes he left?" is a syntactic hybrid. It attempts to re-inflect an idiomatic particle that lost its verbal conjugation over a century ago.