What is the difference between these three sentences and what is the exact meaning of them? In particular, what is the exact meaning of would in the third sentence?
How do you know?
How did you know?
How would you know?
Has would got the same meaning in the following sentences?
Why would you do that?
This would be great.
How would you know?
What would you want?
This is quite a nice answer to your question.
In response to your first question, here's what I would say:
How do you know? - How does the person know the information (present)
How did you know? - How did the person know the information (past)
The above examples mean the same thing, just one is in the present and one is asking about the past.
How would you know? - Probably used in a response/remark, e.g:
A. I think you stole the money
B. How would you know?
You can see in answer B that you can put the stress on would or you, to differ the meaning. E.g:
How would you know? - Asking how the person would know the information
How would you know? - Asking how the person in particular would know
Overall, the difference between do and would in this context is:
How do you know? - this is used when wondering how someone found out: e.g:
A. I know you stole the money B. How do you know?
As if to be admitting to it - the secret is out
and
How would you know? - probably a remark, the person thinks that the other person will not know
A. I know what you were talking about. B. How would you know?
This could be true or false, the person is not replying directly, but is just stating how the person would have found out.
In response to your second question, again, you should look here.
