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Can I say "How so sure?" to question the validity of a claim. For example:

A) They will lose.

B) How so sure?

Is it grammatical? If yes then is it common and natural?

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I think it's more natural for B to say, "Are you sure?", "Why are you so sure?", "Why do you feel so sure?", or "How can you be so sure?" – Damkerng T. May 28 '14 at 19:30
or perhaps: how come you are so sure? – Nico May 28 '14 at 19:51
Thank you both so much :) – GATA May 28 '14 at 20:03
I think "How so sure?" is either a very limited-currency non-standard version of "Why [are you] so sure?", or more likely just an inappropriate form which is more likely to occur to non-native speakers (who it seems to me often conflate how and why). Another "idiomatically normal" response in OP's context might be "You're sure? How's that?" (i.e. - "How does that come about?"). – FumbleFingers May 28 '14 at 20:54
The most common phrase I hear is "What makes you so sure?". – Darthfett May 28 '14 at 21:14
up vote 3 down vote accepted

"Why so sure?" is the form of the question that you're looking for - it is a common structure.

*"How so sure?" is incorrect (though obviously both "How sure?" and "How (come) are you so sure?" are OK - though their meanings are different).

Unless of course you want to know something still else - "How sure are you?" - meaning, are you very sure, or just a little bit sure? (an oxymoron, since certainty would seem to be a binary concept, and yet we can be "sure" and "very sure"...)

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@GATA Actually, I've been wondering that myself... Better yet, "Why sure?" would also be wrong in this meaning. I'll ask over at English.SE - unless you would like to? – Alicja Z May 28 '14 at 20:17
@GATA Ok, posted here... Though from what you've written here so far, it looks like your English is pretty good :) – Alicja Z May 28 '14 at 21:03
@GATA: "Why so sure?" is a common idiom, despite not being formally correct. "How so sure?" is not. There's no good reason, that's just the way it is. – keshlam May 28 '14 at 21:15
@GATA Since you were the co-author of the question on English.SE in a way, does Jon's answer there satisfy you? – Alicja Z May 28 '14 at 22:42
How sure? means how sure are you?, not how are you sure? – Esoteric Screen Name May 29 '14 at 1:58

You don't really have a verb in that sentence. You would probably say "How are you so sure?"

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