how do you say the following in past tense:
( He asked,"how did you know?"
I'm not sure what you mean, since it's all in the past tense already.
"He asked" is straight forwardly in the past tense.
"How did you know" is also in the past tense, but looks a little more confusing because "know" is in the present. However, the "did," which is in the past tense, is what makes the whole thing in the past tense. Other examples of this include:
"How did you make it?" ------- (did + present tense)
"I made it with black pepper"
"You're telling me that you won?"
"I DID win! I'm not fibbing!" ------- (did + present tense)