How did you know? or How do you know?
A: How did/do you know I'm here?
B: Where else would you be?

When do you use "did" and when "do"? Please give some example. Can I just use "did" for everything because the "knowing" happened just now for every situation, thus, past tense?

Thank you.

Question

How did you know? or How do you know?
A: How did/do you know I'm here?
B: Where else would you be?

When do you use "did" and when "do"? Please give some example. Can I just use "did" for everything because the "knowing" happened just now for every situation, thus, past tense?

Thank you.

Answer

The most natural way to say this would be "How did you know I was here?", using the past tense. This is because it refers to the time BEFORE you arrived or before they saw you.

It's pretty obvious how the other person knows that you're here now, in present time - you're standing right in front of them! What you want to know is how come they expected you to be there, even before they saw you.

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