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It may seem a rather strange question, however, not all foreign learners can understand and interpret the phrase (or part of it) "How do you know?" correctly.

I mean, when we ask "How big is it?" - we mean "to what extent". When "How is it done?" - we mean "in what way". It usually always has a straight interpretation in a straight context. Nevertheless, the phrase "How do you know that? " for instance, can often mean different things even in one context. As far as I understand, however, it should always mean "How come have you obtained knowledge about that?".

If that is so, then does it matter whether it is "Who told you that?", "Where did you find out about that?", "How did you find out about that?" or pure expression of unbelievabilty "Why on earth do you know about that?"

  • There are multiple means of knowing something, the means of knowing something is how you know it: through reading, talking to others, finding something, seeing, etc. How = means. And the means are many. Whether something is believable is another issue, as I see it. – Lambie May 10 '18 at 15:26
  • How come = why. How = via what means, method, and opportunity. – Tᴚoɯɐuo May 10 '18 at 16:35
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"How do you know that?" is not an ambiguous phrase. The problem is the one pronoun that which could represent various things, depending on the context.

Example:

A. Robert stole the money from the safe.
B. How do you know that?
A. Do you mean, how do I know the money was stolen, or how do I know Robert was the one who stole it? Or are you asking how I found out about the theft in the first place?

If it's not clear, you should ask for more information. If you can't ask -- for example, if it's in a movie or a book, then you make your best guess from the context.

A. General! Reports indicate the enemy is shifting their troops to the east!
B. How do we know that?
A. Sir! Our observation posts from the eastern trenches report large troop movements through their area!

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how come wants to know why.

How come Johnny gets to stay up late and I have to go to bed?
-- Because he's 10 and you're only 5.

How come you're taking the train today?
-- Because my car's in the shop.

And so these questions

How come you know that?

How do you know that?

can be very different questions, though they can mean much the same, depending on emphasis when spoken.

How come you know that? can be asking why you, in particular, should have this knowledge. It may strike the speaker as odd that you should be in possession of this knowledge.

How do you know that? can be asking for your source, or for the authority on which you're basing your statement. What makes you so certain?

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